Russian Foreign Minister Sergei Lavrov said today that any US military strikes against the Syrian government army would complicate attempts to fight Islamic State and other groups.
The United States said yesterday it had decided to allow airstrikes to defend Syrian rebels trained by the US military from any attackers.
"We say that this position is a violation of international law and represents an obstacle on the road to forming a united front to fight terrorism, including Islamic State and al-Nusra Front," Mr Lavrov told a news conference with Qatari Foreign Minister Khalid al-Attiyah.
His comments were translated into Arabic by al-Jazeera television.
Russia has been a principle supporter of Syrian President Bashar al-Assad during Syria's four-year-old war, offering military supplies and diplomatic support.
The conflict began as a pro-democracy uprising but hardline Islamist militants have emerged as the strongest rebel factions opposing President Assad, complicating the United States and other Western governments' support for opposition forces.
Mr Lavrov, who also met with US Secretary of State John Kerry and Saudi Foreign Minister Adel al-Jubeir in Doha today, said a settlement to the Syrian war needed dialogue between all parties.
But Islamic State was the main danger in Syria and neighbouring Iraq and that was why Moscow supported the governments of both countries, he said.
"We provide military and technical support to the Syrian government to fight this danger (Islamic State), just as we provide this support to Iraq to fight Islamic State," Mr Lavrov said, according to al Jazeera.
Russia has been trying to bring about rapprochement between the Syrian government and regional states including Saudi Arabia and Turkey to forge an alliance to fight Islamic State, which has seized control of large amounts of Syrian territory.
A senior US official said that in their meeting, the ministers "acknowledged the need for a political solution to the conflict and the important role to be played by opposition groups in reaching that solution".