The US accuses Mr Assad's forces of killing 1,429 people in a sarin gas attack on 21 August.

Repeating a phrase he used earlier in the week, Mr Kerry said the international community was facing a "Munich moment" - a reference to the policy of appeasement that failed to stop Nazi Germany in the 1930s.

"We in the United States know, and our French partners know, that this is not the time to be silent spectators to slaughter," he said.

He insisted there was growing support for Washington's call for intervention in Syria, saying: "There are a number of countries, in the double digits, who are prepared to take military action."

This was more countries than could actually be used "in the kind of military action being contemplated", Mr Kerry added.

Mr Fabius - who staunchly backs Mr Kerry on this issue - added that there was "wide and growing support" for action.

Earlier on Saturday, in the Lithuanian capital Vilnius, Mr Kerry welcomed a statement on Syria by EU foreign ministers who were meeting there.

The EU ministers urged a "clear and strong response" to the alleged chemical attack.

But the EU also welcomed French President Francois Hollande's call to wait for the UN weapons inspectors' report before taking any further action.

Mr Hollande said he expected the report to be ready by next weekend.

Meanwhile the BBC has learnt that the UK government has sent chemical protection suits to some members of the opposition forces in Syria this week, as it continues to give technical and non-lethal aid to members of the Syrian national coalition.

Mr Kerry's visit to Europe comes amid deep divisions over whether to take military action in Syria.

The G20 summit in Russia last week failed to produce international agreement, with US President Barack Obama at odds with Russia's President Vladimir Putin, who blames the gas attack on rebels.

Both Russia and China, which have refused to agree to a UN Security Council resolution against Syria, insist any military action without the UN would be illegal.

President Obama now faces a tough week of trying to persuade Congress to authorise military action.

He has only a few days to convince Congress, which returns from its summer recess on Monday. Both the Senate and House of Representatives could vote on the Syrian issue later this week.

A poll commissioned by the BBC and ABC News suggested more than a third of Congress members were undecided whether or not to back military action - and a majority of those who had made a decision said they would vote against the president.

Many remain concerned that military action could draw the US into a prolonged war and spark broader hostilities in the region.

Some 100,000 people have already been killed in the two-and-a-half-year-old Syrian conflict, according to the UN.