The two planes jettisoned four bombs - two inert and two unarmed - in more than 50m (165 ft) of water, away from coral, to minimise damage to the World Heritage Site, the US navy said.

The jets were to bomb an island range nearby, but the mission was aborted.

The AV-8B Harriers were low on fuel and could not land loaded, the navy added.

The incident happened during the training exercise Talisman Saber, involving US and Australian military personnel.

The two jets had been instructed to target the bombing range on Townshend Island.

However, the mission was aborted when hazards were reported in the area.

"It was not safe to drop the bombs. There were civilian boats right below them," Commander William Marks of the US Seventh Fleet told ABC radio.

The planes then dropped the bombs in the marine park off the coast of Queensland. None of the devices exploded.

The bombs - each weighing 225kg (500lbs) - do not pose a hazard to shipping, the US Navy says, and charts are to be updated to show the unexploded ordnance.

"There is minimal environmental impact," said Cmdr Marks. "It is a safe situation for the environment, for shipping, for navigation."

However, the US Navy said it was open to the idea of recovering the bombs from the seabed.

"If the [Great Barrier Reef] park service and the government agencies of Australia determine that they want those recovered, then we will coordinate with them on that recovery process," said Seventh Fleet spokesman Lt David Levy.

The Great Barrier Reef is the world's largest coral structure rich in marine life.

It stretches for more than 2,600km (1,680 miles) along Australia's eastern coast.

The earthquake near Dingxi city had a magnitude of 5.98 and was shallow, with a depth of just 9.8 km (6 miles), according to the US Geological Survey.

Dingxi local authorities say many houses have collapsed in the quake.

In 2008, an earthquake in Sichuan province left up to 90,000 people dead and millions homeless.

At least 47 people had been killed, reports from China's national radio and Agence-France Presse news agency citing local officials said.

Another 296 other people were injured, the Dingxi local government said on its official Sina Weibo microblog.

In Gansu's Zhangxian county, at least 5,600 houses were seriously damaged and 380 collapsed, while some areas suffered from power cuts or mobile communications being disrupted, the Earthquake Administration of Gansu province said.

Crews of fire fighters and rescue dogs have already arrived at the scene, the BBC's Celia Hatton in Beijing reports.

The closer to the surface an earthquake strikes, the more damage it can cause, our correspondent adds.

The earthquake reportedly triggered a series of mudslides and landslides, state-run news agency Xinhua reported.

The Gansu military police has deployed 500 soldiers, including 120 specialist rescuers, while 500 emergency tents and 2,000 quilts are also being transported to affected areas, Xinhua added.

Officials from the civil affairs, transportation and earthquake departments are also visiting local towns to assess the damage, a statement on the Dingxi party website said.

"You could see the chandeliers wobble and the windows vibrating and making noise, but there aren't any cracks in the walls," AFP quoted a clerk at Wuyang Hotel, about 40 km (25 miles) from the epicentre, as saying.

"Shop assistants all poured out onto the streets when the shaking began," the clerk said.

The 6.5-magnitude tremor was centred 35 miles (57 km) off the coast south of the capital at a depth of 6.3 miles, said the US Geological Survey.

But while some structural damage and power cuts were reported, officials said there was no risk of a tsunami.

The quake hit at 17:09 (05:09 GMT) and was felt as far north as Auckland.

It smashed windows, knocked stock off shop shelves and burst some water pipes, but there have been no reports of serious casualties.

Wellington resident James Mclaren said the earthquake had caused power cuts in the city suburbs and prompted the temporary closure of its airport.

"There's been a bit of structural damage, lots of shattered glass everywhere," he told the BBC. "Initially there were a few screams and panic, people thought it was another Christchurch."

A 6.3-magnitude earthquake centred near Christchurch in February 2011 killed 185 people.

Sunday's tremor was the latest in a series that have shaken the lower half of New Zealand's North Island in recent days.

New Zealand lies on the notorious Ring of Fire, the line of frequent quakes and volcanic eruptions that circles virtually the entire Pacific rim.

The country experiences more than 14,000 earthquakes a year, of which only around 20 have a magnitude in excess of 5.0.

Satellite images of Africa's Congo Basin reveal that deforestation has fallen by about a third since 2000.

Researchers believe this is partly because of a focus on mining and oil rather than commercial agriculture, where swathes of forest are cleared.

The work is published in the Philosophical Transactions of the Royal Society B.

It is part of a series that is examining the state of Africa's forests.

Dr Simon Lewis, from the University of Leeds and University College London, said: "Most of the focus has been on the Amazon and on South East Asian tropical rainforests, and a big bit of the missing picture is what is going on the Congo Basin in Central Africa.

"We really wanted to pull together information about this amazing region that we know very little about."

Bigger trees

Sprawling across the heart of Africa, the Congo Basin rainforest is second only to the Amazon in size. It covers nearly 2 million sq km (800,000 sq miles).

This latest study reveals that it is in far better health than expected.

Images taken from space allowed researchers to track how the dense foliage was changing over time.

They found that during the 1990s nearly 3,000 sq km (1,000 sq miles) of forest were being felled each year.

But from 2000 to 2010, the rate of deforestation had slowed. Fewer than 2,000 sq km (700 sq miles) of rainforest were lost every year.

"The results were surprising," Dr Lewis told BBC News.

"This is partly because there is a network of protected areas. But it is also because of a lack of expansion from agriculture, and the way these [central African] countries have organised their economies.

"They are very dependent on oil sales and also minerals from mining, and they are investing in that and not investing so much in agricultural expansion."

Another study published in the same journal highlighted the differences between trees in the Congo Basin and those found elsewhere around the world.

Measurements of 130,000 trees show that African species were on average much larger in stature than those in the Amazon. As a result, scientists believe that the Congo Basin stores more carbon, acting as a carbon sink.

Losing parts of the rainforest would have an impact on climate change, biodiversity and the communities that depend on the environment, said Dr Lewis.

He said that Africa's rainforest was at a pivotal point.

Logging could continue to fall because of schemes such as the United Nations' Reducing Emissions from Deforestation and Forest Degradation in Developing Countries (REDD) that offer financial incentives to keep forests intact.

"But on the other side, the big increase in human population and the rise in living standards globally means we may need more agricultural commodities... It could go the other way and go much more like South East Asia or the Amazon and see the expansion of commercial agriculture," explained Dr Lewis.

A statement issued earlier supports the automatic exchange of tax information between countries.

It also backs plans by the Organisation for Economic Cooperation and Development to stop firms moving their profits across borders to avoid taxes.

The OECD said some firms "abuse" current rules to avoid tax.

UK Chancellor George Osborne said the announcement, which came after a two-day G20 meeting in Moscow, was an "important step towards a global tax system that is fair and fit for purpose for the modern economy".

Last month, the G8 group of leading economies agreed a deal to "fight the scourge of tax evasion", and nations including the UK, France, Germany, the USA and Australia are taking part in a pilot information exchange scheme.

British Prime Minister David Cameron made the issue a priority for the UK's presidency of the G8 this year, and Australia has agreed to do the same during its G20 presidency next year.

The OECD said current tax rules, some dating to the 1920s, were created to avoid "double taxation" of companies working in more than one country - but it said they were being abused to allow "double non-taxation".

BBC business correspondent Joe Lynam said the "bandwagon of clamping down on aggressive tax avoidance" was moving on from developed economies to emerging ones like Brazil and India.

The rules should mean bigger bills for companies which could previously "pit one country off against another in terms of tax", our correspondent added.

The G20 asked the OECD to come up with a plan to improve tax cooperation, and the finance ministers said they "fully endorse the OECD proposal for a truly global model" of information sharing.

Their statement called on all countries to make automatic information sharing a reality "without further delay", adding that "capacity-building support" would be provided for poorer nations.

Closing loopholes

The G20 said the changes should be in place within two years, but our correspondent called that "very ambitious" because hundreds of tax treaties exist between countries and "thousands of amendments" might be needed.

Many multinational firms currently avoid tax - legally - by means including loopholes and tax havens, but the new rules could require them to pay more in the countries where they do business.

Firms including Google, Starbucks, Amazon and Apple have been criticised for the amount of tax they pay.

Earlier this year, MPs attacked Google for routing £3.2bn of UK sales through Dublin and paying little tax as a result.

Starbucks has been questioned for transferring money to a Dutch sister company in royalty payments, though the firm agreed to pay more tax after strong public criticism.

The companies point out that these schemes are legal and they have a duty to shareholders to minimise their tax bills.

The UK prime minister told the BBC there was a "stalemate" on the ground, but work must continue internationally to try to find a solution.

UK military chiefs have warned of the risks of arming rebel groups.

Mr Cameron said there was "too much extremism" among the opposition, but moderate groups still deserved support.

Syrian government forces have taken the initiative in recent months, and have been bolstered by the capture of the strategically important town of Qusair in the west of the country in June.

Most of the much bigger city of Homs has been recaptured by government troops backed by Lebanon's Hezbollah.

Washington and London have been looking at ways to increase logistical support for opposition groups and the UK announced last week it was sending £650,000-worth of protective clothing to guard them against chemical and biological attacks.

But reports have suggested support for supplying weapons to rebel groups is receding, due to strong political opposition and widespread concerns about arms falling into the wrong hands.

Mr Cameron told the Andrew Marr show that President Bashar al-Assad was an "evil" man who was doing "terrible things to his people".

But he also said the UK "should have nothing to do" with elements of the opposition also reported to have committed atrocities.

"It is a very depressing picture and it is a picture which is on the wrong trajectory," he said of the conflict.

"There is too much extremism among the rebels. There is also still appalling behaviour from this dreadful regime using chemical weapons. There is an enormous overspill of problems into neighbouring countries."

He added: "I think he [Assad] may be stronger than he was a few months ago but I'd still describe the situation as a stalemate."

'Head in sand'

Despite the UK's concerns about the actions and views of some opposition groups, Mr Cameron said the UK had a duty to support those pushing for a "democratic, free and pluralistic" country.

"You do have problems with part of the opposition which is extreme, that we should have nothing to do with. But that is not a reason for pulling up the drawbridge, putting our head in the sand and doing nothing.

"What we should be doing is working with international partners to help the millions of Syrians who want to have a free democratic Syria, who want to see that country have some form of success."

Mr Cameron also said reports in the media that his wife had been a strong influence on his policy were a "total urban myth".

Samantha Cameron visited a refugee camp in Lebanon in March in her role as ambassador for Save The Children, in which she met families and children displaced by the conflict.

It has been suggested that she has since pressed for a stronger humanitarian response.

The UN says the refugee crisis is the worst for 20 years, with 1.7 million forced to seek shelter in neighbouring countries and an average of 6,000 people fleeing every day this year.

Mr Cameron said his wife had been "very moved" by what she had seen and heard from people who had lost loved ones and whose communities had been destroyed.

But he added: "She does not influence my policy on this. I have been very passionate about this for a long time."

The UK prime minister told the BBC there was a "stalemate" on the ground, but work must continue internationally to try to find a solution.

UK military chiefs have warned of the risks of arming rebel groups.

Mr Cameron said there was "too much extremism" among the opposition, but moderate groups still deserved support.

Syrian government forces have taken the initiative in recent months, and have been bolstered by the capture of the strategically important town of Qusair in the west of the country in June.

Most of the much bigger city of Homs has been recaptured by government troops backed by Lebanon's Hezbollah.

Washington and London have been looking at ways to increase logistical support for opposition groups and the UK announced last week it was sending £650,000-worth of protective clothing to guard them against chemical and biological attacks.

But reports have suggested support for supplying weapons to rebel groups is receding, due to strong political opposition and widespread concerns about arms falling into the wrong hands.

Mr Cameron told the Andrew Marr show that President Bashar al-Assad was an "evil" man who was doing "terrible things to his people".

But he also said the UK "should have nothing to do" with elements of the opposition also reported to have committed atrocities.

"It is a very depressing picture and it is a picture which is on the wrong trajectory," he said of the conflict.

"There is too much extremism among the rebels. There is also still appalling behaviour from this dreadful regime using chemical weapons. There is an enormous overspill of problems into neighbouring countries."

He added: "I think he [Assad] may be stronger than he was a few months ago but I'd still describe the situation as a stalemate."

'Head in sand'

Despite the UK's concerns about the actions and views of some opposition groups, Mr Cameron said the UK had a duty to support those pushing for a "democratic, free and pluralistic" country.

"You do have problems with part of the opposition which is extreme, that we should have nothing to do with. But that is not a reason for pulling up the drawbridge, putting our head in the sand and doing nothing.

"What we should be doing is working with international partners to help the millions of Syrians who want to have a free democratic Syria, who want to see that country have some form of success."

Mr Cameron also said reports in the media that his wife had been a strong influence on his policy were a "total urban myth".

Samantha Cameron visited a refugee camp in Lebanon in March in her role as ambassador for Save The Children, in which she met families and children displaced by the conflict.

It has been suggested that she has since pressed for a stronger humanitarian response.

The UN says the refugee crisis is the worst for 20 years, with 1.7 million forced to seek shelter in neighbouring countries and an average of 6,000 people fleeing every day this year.

Mr Cameron said his wife had been "very moved" by what she had seen and heard from people who had lost loved ones and whose communities had been destroyed.

But he added: "She does not influence my policy on this. I have been very passionate about this for a long time."