In an interview with state-run Channel One television ahead of the G20 summit in Saint Petersburg this week, Putin warned the West that military action against the Syrian regime without UN Security Council backing would be considered an "aggression".
Asked whether Russia would agree with US-led military strikes if it was proven that the Syrian regime had carried out the chemical attack outside Damascus on August 21, Putin replied: "I do not exclude that."
But he said the West still needed to put forward watertight proof of the circumstances of the attack, which some Russian officials have blamed on rebels.
"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," Putin said.
But he said it would be unacceptable for the West to go ahead with military action against the regime of Bashar al-Assad without the assent of the UN Security Council, where Russia has veto-wielding permanent membership.
"Only the UN Security Council can give approval for the use of force against another state," Putin said.
"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," Putin said.
In an interview apparently aimed at presenting a more pragmatic face to the world ahead of the G20 summit, Putin confirmed that Russia had delivered some components of sophisticated S-300 missile systems to Syria.
But he revealed for the first time deliveries had now been halted.
"We have delivered separate components but the whole delivery has not been completed and for the moment we have suspended it," Putin said.