Moscow warned on Wednesday that any US Congress approval for a military strike against Syria without UN approval would represent an "aggression" and that such action could have far-reaching nuclear security implications.

The Kremlin's stark language came as it demanded "convincing" proof that the regime of Bashar al-Assad was responsible for using chemical weapons against its own people.

Russian President Vladimir Putin said it would be unacceptable for the West to go ahead with military action against Damascus without UN Security Council approval, according to Russian news agencies.

"Only the UN Security Council can give approval for the use of force against another state," Putin told members of the board of human rights in the Kremlin prior to a meeting of the G20 in Saint Petersburg.

"Any other ways to justify the use of force against another sovereign and independent state are unacceptable and cannot be qualified as anything other than aggression.

"But Syria, as we know, does not attack the United States, it can therefore be no question of defence."

Earlier in the afternoon the Russian president suggested Russia could approve military strikes against Syria -- if the West presented watertight evidence of chemical weapons crimes.

Putin said the West needed to put forward cast-iron proof of the circumstances of the attack, which some Russian officials have previously blamed on rebels seeking to discredit the regime.

"If there is evidence that chemical weapons were used, and by the regular army,... then this evidence must be presented to the UN Security Council. And it must be convincing," Putin said.

If there was clear proof of what weapons were used and who used them, Russia "will be ready to act in the most decisive and serious way", he said.

However, on Wednesday evening, Russia's foreign ministry warned that military intervention in Syria could have "catastrophic" consequences for nuclear security.

"The escalation of the situation around Syria is of particular concern since an action against the country could reach sensitive targets from the viewpoint of nuclear safety," said a statement from ministry spokesman Alexander Lukashevich.

"If weapons fell on a mini-reactor in the suburbs of Damascus it could have catastrophic consequences: a possible contamination of land by highly enriched uranium and other radioactive materials," he said, adding it would be "impossible to guarantee control of nuclear material".

The United States has indicated it is prepared to go ahead with military action without UN approval, but President Barack Obama is first seeking approval from Congress which will push back the timetable until after the G20 meet which begins Thursday.

Speaking in Sweden ahead of travelling to the G20, Obama said he hoped Putin would change his position on Syria.

Secretary of State John Kerry and Pentagon chief Chuck Hagel on Tuesday in the Senate defended Obama's announcement of "limited" punitive strikes on Syria, but not before getting a green light from Congress.

The US, along with France, is trying to establish an international coalition for a military response against Damascus, which it holds culpable for a chemical weapons attack that killed hundreds in Damascus suburbs on August 21.

The conflict has so far claimed 110,000 lives, according to NGOs, and the UN estimates two million Syrian refugees have left the country.