Syrian officials supplied the evidence, which Mr Lavrov has not yet seen.

A UN report released on Monday concluded the nerve agent sarin was used in the attack in Damascus, in which hundreds were killed.

The US blamed government forces for the attack, but Russia and Damascus have insisted that rebels were responsible.

The UN report did not apportion blame for the attack, which sparked diplomacy that culminated in a deal for Syria to hand over its chemical arsenal by mid-2014.

The report's author, Ake Sellstrom, told the BBC he believed that the task of finding and destroying Syria's chemical stockpile would be "stressful work", but was "doable".

He said much depended on whether the Syrian government and the opposition were willing to negotiate.

The disarmament deal was brokered by the US and Russia.

The penalty for any possible breaches by Syria are now being thrashed out by the UN Security Council permanent members.

Mr Lavrov said there was plenty of evidence that pointed to rebel involvement in chemical attacks similar to the Damascus attack.

"We will have to find out who did it," he said.

Earlier Mr Lavrov's deputy, Sergei Ryabkov, said he had been given the evidence during a trip to Syria.

He said it needed to be analysed, and gave no details of its content.

Mr Ryabkov criticised the UN report, saying it was "distorted" and "one-sided".

"The basis of information upon which it is built is not sufficient, and in any case we would need to learn and know more on what happened beyond and above that incident of 21 August," he said.

"We are disappointed, to put it mildly, about the approach taken by the UN secretariat and the UN inspectors, who prepared the report selectively and incompletely."

In response to Mr Ryabkov's comments, Mr Sellstrom told the BBC he thought Russia was not criticising the report itself but the process, which he described a political matter and therefore not his remit

French Foreign Minister Laurent Fabius defended the UN report, saying he was surprised by the Russian reaction.

"Nobody can question the objectivity of the people appointed by the UN," he said.

Human Rights Watch has taken the trajectory of the rockets from the UN document and plotted their likely path.

The rights group said the likely launch site for the missiles was in a government military compound.

The UN inspectors were originally mandated to go to Syria to investigate three alleged chemical weapons attacks, at Khan al-Assal, Sheikh Maqsoud and Saraqeb.

But they were later ordered to shift their focus to the Damascus incident, which was the most deadly chemical assault.

They are due to return to Syria "within weeks" to complete their inquiry into the other attacks, and a report is due in October.

On Tuesday the five permanent UN Security Council members met in New York to discuss a draft resolution put forward by the UK, France and the US.

They want a resolution containing the threat of military action against Syria if it fails to comply with the disarmament deal, but Russia opposes this.

A resolution under Chapter VII of the UN charter permits military action if other measures do not succeed. Chapter VI requires a purely negotiated solution.

The BBC's Daniel Sandford in Moscow says Russia has delivered a promise from Syria to give up its chemical weapons, and it seems that at this stage Moscow does not feel like giving the Western allies anything more.

Russia and China have three times blocked Western-backed Security Council resolutions against Mr Assad.

More than 100,000 people have died since the uprising against President Assad began in 2011.

Millions of Syrians have fled the country and millions more have been internally displaced.

LUKA: Thirty-seven people were killed when a fire swept through a wooden psychiatric hospital in northwest Russia overnight Thursday, in the latest tragedy to hit the country's mental health institutions.

The fire was started by a patient who was either smoking or deliberately set fire to his bed at the hospital in the village of Luka, 220 kilometres (137 miles) southeast of Saint Petersburg, officials said.

The institution was made out of wood and officials said it had been previously warned by the judicial authorities to improve its fire security.

A criminal probe has been opened into suspected negligence.

"During a fire in the Oksochi psychiatric hospital 37 people died," regional investigators said in a statement, adding that 10 people have already been pulled from the debris.

"A 46-year-old nurse who was saving people perished in the fire," investigators added.

The fire, which broke out in the middle of the night, reduced the aged wooden building on the outskirts of the village to smouldering wreckage.

Rescue teams were combing through the debris, and bodies in black plastic bags were taken away from the scene.

Novgorod region governor Sergei Mitin told AFP at the scene that 23 people had been rescued.

Ilya Denisov, a representative of the emergencies ministry, did not rule out that more survivors could be found.

He said the firefighters were quick to react but that by the time they arrived the fire had consumed the entire building.

Local authorities said the hospital housed patients with grave psychological disorders, making evacuation even more complicated.

Lidiya Vasilyeva, 40, who lives near the hospital, said the fire broke out around 2:00 am.

"The fire spread very quickly," she said. "Many patients were pushed out of the windows, many did not want to come out or did not come out immediately, it was very scary."

She said it was difficult to pinpoint the cause of the blaze but suggested a patient might be at fault.

"In the summer they brought a patient here who they say was suffering from pyromania," she told AFP.

Investigators said they had opened a criminal probe into suspected negligence causing death.

"According to preliminary information, one of the patients set fire to himself and his bed," said the investigative committee, a Russian law enforcement agency that probes major cases.

Officials said concerns had already been aired about the state of the building, and Russian state television said the authorities had been planning to close down the hospital.

"This building had a weak resistance to fire. It was made of wood. The administration had been told by the legal authorities to remedy numerous violations in fire security by August 1," said the head of oversight at the emergencies ministry, Yuri Deshovykh.

"But this was not done," he was quoted as saying.

The Kremlin's human rights envoy, Vladimir Lukin, sounded the alarm over the state of psychiatric hospitals in the country, calling for a joint effort to improve oversight.

"The entire society, people should protect citizens who have found themselves in a unique, difficult situation when they cannot protect themselves," Lukin said on popular radio station Moscow Echo.

The fire was the latest tragedy to hit a medical institution in Russia, which struggles with outdated Soviet-era infrastructure and lax security procedures.

In April, a fire that ravaged a psychiatric hospital in the Moscow region killed 38 people, most of them patients engulfed by flames as they slept behind barred windows.

In 2006, a fire in a Moscow drug rehabilitation clinic killed 45 women, many of whom were trapped by metal bars on the windows that staff could not open.