Russian President Vladimir Putin has challenged the US to present to the UN evidence that Syria was responsible for chemical weapons attacks.
Putin said it would be "utter nonsense" for Syria's government to provoke opponents with such attacks when it was in a position of strength.
US President Barack Obama has said he is considering military action against Syria based on intelligence reports. Putin's remarks came after UN weapons inspectors had left Syria.
They crossed into neighbouring Lebanon after four days of inspections, including investigations of what happened in the Damascus suburbs on 21 August.
Hundreds of people including children were killed in the suspected chemical weapons attacks, which the US says was carried out by the Syrian government.
Syria said the US claim was "full of lies", blaming rebels for the attacks.
The inspectors' departure from Syria removes both a practical and a political obstacle to the launch of US-led military action, says the BBC's Kevin Connolly in Lebanon.
Barack Obama: "We're not considering any boots on the ground approach"
But speaking to journalists in the Russian far-eastern city of Vladivostok, Putin urged Obama - as a Nobel Peace Prize laureate - to think about future victims in Syria before using force.
He said it was ridiculous to suggest the Syrian government was to blame for the 21 August attack.
"Syrian government troops are on the offensive and have surrounded the opposition in several regions," he said.
"In these conditions, to give a trump card to those who are calling for a military intervention is utter nonsense."
Russia - a key ally of Syria - has previously warned that "any unilateral military action bypassing the UN Security Council" would be a "direct violation of international law".
Moscow, along with China, has vetoed two previous draft resolutions on Syria.
Putin also expressed surprise at a vote in the British parliament on Thursday ruling out participation in military action.
"I will be honest: this was completely unexpected for me," he said.
"This shows that in Great Britain, even if it is the USA's main geopolitical ally in the world... there are people who are guided by national interests and common sense, and value their sovereignty."
Meanwhile in France - seen as the main US ally since the UK vote - an opinion poll suggested that 64% opposed the use of force.
Neither France nor the US needs parliamentary approval for military action.
Syrian President Bashar al-Assad has said his country will defend itself against any Western "aggression".