Russia on Friday condemned a strike by the US-led coalition against Syrian pro-regime forces as "illegitimate" and said it could harm efforts to fight Islamic State jihadists.
"It is illegitimate, unlawful and yet another gross violation of Syria's sovereignty," Russian Foreign Minister Sergei Lavrov said in televised comments while on a trip to Cyprus.
He added that Moscow was "concerned" that the "emerging understanding" of the need to unite efforts against the Islamic State group and the former Nusra front is "starting to erode".
US-led warplanes on Thursday carried out a strike in the east of the country against a convoy of pro-government forces which were heading towards a remote coalition garrison near the border with Jordan.
In a statement, the coalition said it had struck "pro-regime forces... that posed a threat to US and partner forces."
The coalition said the strike came after unsuccessful "Russian attempts to dissuade Syrian pro-regime movement" as well as "a coalition aircraft show of force, and the firing of warning shots."
Lavrov said he was "unaware" that Russia had been given any warning of the strike, and said preliminary information suggested there were civilian casualties. "We are still confirming the details but according to some reports several dozen civilians died as a result of this strike," he was reported as saying by Russian news wires. His comments echoed an earlier statement by a deputy, Gennady Gatilov, who said the coalition's strikes on Syrian forces were "unacceptable" and could hinder peace talks.
Russia and Iran have deployed forces to Syria to back up President Bashar al-Assad while the US and many of its regional and Western allies have called for the strongman to leave power.
The United States infuriated the Kremlin in April by bombing a Syrian airbase in response to an alleged chemical attack by Damascus.
Representatives of Syria's government and the opposition High Negotiations Committee are in Switzerland for the sixth round of UN-backed peace talks, but there has been little sign of progress. In the first concrete results from the talks this week, the United Nations said the warring sides had agreed to set up expert committees to discuss "constitutional issues".
Another deputy foreign minister, Mikhail Bogdanov, told the TASS news agency that Russia "supports" the move.