Moscow is warning that any US Congress approval for a military strike against Syria without UN approval will 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 says it will 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 St 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 says the West needs 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 is clear proof of what weapons have been 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".
