Moscow has accused Washington of prolonging the Syrian conflict by supporting the opposition, on the eve of a UN Security Council vote that threatens to further deepen big power divisions.
On the ground, a car bomb exploded at a border crossing between Syria and Turkey as rebels battled to prevent regime forces from seizing their last stronghold in the strategic Qalamun region.
Speaking in Baghdad, Russia's Foreign Minister Sergei Lavrov said America's policy on Syria "encourages extremists who are financing terrorism and supplying terrorist organisations and groups with weapons".
"In the end, this will not result in anything except the escalation of the Syrian conflict," he said in translated remarks.
Moscow is a key backer of President Bashar al-Assad's regime, which has been battling an uprising that began in March 2011.
It supplies the government with weapons and provides diplomatic cover at the United Nations, where the Security Council is expected to vote Friday on a resolution on humanitarian aid for Syria.
The draft resolution was presented on Thursday but does not so far have Russian backing.
It calls on all parties to end sieges of densely populated areas immediately, and to stop all attacks on civilians.
It also wants all parties to authorise humanitarian groups to deliver aid across front lines and borders.
The demands are addressed to both sides in the war, but especially to the government, as the text said it is mainly responsible for protecting civilians.
The resolution has no clause allowing for sanctions in case of non-compliance.
A source in The Hague, meanwhile, said Syria is likely to miss a UN-backed June 30 deadline to destroy its chemical arsenal, possibly by months.
An Organisation for the Prohibition of Chemical Weapons meeting on Friday is expected to hear calls for Syria to do more.
On the ground, meanwhile, the Syrian Observatory for Human Rights said a car bomb exploded at the Bab al-Salama crossing between Syria and Turkey.
The Observatory said at least five people were killed, while a Turkish official told AFP four people were killed and 45 wounded.
