Moscow accused Washington of prolonging the Syrian conflict by supporting the opposition ahead of a UN Security Council vote Saturday 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.
And Syria's government media said Jordan was seeking to stir up the southern front in the country's conflict after a stalemate in peace talks held in Geneva this month.
Speaking in Baghdad on Thursday, 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 remarks translated from Russian into Arabic.
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 officials said the Security Council is to vote Saturday on a resolution on humanitarian aid for Syria.
Russia, with support from China, has blocked three previous resolutions aimed at pressuring the Damascus regime since the crisis began, with an estimated 250,000 people across Syria awaiting help.
The resolution calls on all parties to immediately provide humanitarian access to populated areas, including the besieged city of Homs.
It also demands the "the immediate cessation of all attacks against civilians", such as through the use of barrel bombs, a clear reference to Syrian army tactics used in Aleppo, in the north.
And it calls for all parties, in particular the Syrian regime, to authorise humanitarian groups to deliver aid across front lines and borders.
Humanitarian groups have been seeking cross-border access for some time to allow aid to be shipped directly into Syria from neighbouring countries such as Iraq or Turkey.
Moscow's UN Ambassador, Vitaly Churkin received the text late Wednesday and requested time to consult his government.
The resolution was submitted by Australia, Luxembourg and Jordan, and is backed by Britain, the US and France.
A source in The Hague, meanwhile, said Syria was likely to miss a UN-backed June 30 deadline to destroy its chemical arsenal, possibly by months.
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An Organisation for the Prohibition of Chemical Weapons (OPCW) meeting on Friday is expected to hear calls for Syria to do more amid growing Western frustration with Damascus' perceived delays.
- Car bomb hits border post -
On the ground, the Syrian Observatory for Human Rights said a car bomb exploded at the Bab al-Salama crossing between Syria and Turkey.
A Turkish official told AFP six people were killed and 45 wounded, with the injured taken to hospital in Turkey's Kilis province.
In Damascus province, rebels engaged in fierce clashes with regime forces backed by the pro-government National Defence Forces militia and fighters from Lebanon's Hezbollah in Yabrud.
The town is the last remaining stronghold of the opposition in the strategic Qalamun region near the border with Lebanon.
It sits on the main highway between Damascus and the country's third city Homs, and has been a key bastion for the opposition since early in the uprising.
The battle is also important for Hezbollah, the powerful Shiite movement, which says many of the explosive-packed cars that have blown up in attacks targeting its Lebanese strongholds came from Yabrud.
In the northern city of Aleppo, eight soldiers were killed by a rebel suicide bombing at the city's central prison, the Observatory said.
And 18 rebels were killed in a dawn assault on an Alawite village in central Homs province.
Syria's government daily Al-Thawra, meanwhile, accused Jordan of trying to stir up the southern front of the uprising, where rebels say they are preparing for a spring offensive against Damascus.
It said Jordan's efforts came after the stalemate at a second round of peace talks in Geneva this month.
"As the saying goes, those who play with fire will get burned, and so what then about those who start fires," the daily wrote.