Russian Foreign Minister Sergei Lavrov said on Monday that any U.S. military strikes against the Syrian government army would complicate attempts to fight Islamic State and other groups.
The United States said on Sunday it had decided to allow air strikes to defend Syrian rebels trained by the U.S. 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," 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 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 Assad, complicating the United States and other Western governments' support for opposition forces.
Lavrov, who also met with U.S. Secretary of State John Kerry and Saudi Foreign Minister Adel al-Jubeir in Doha on Monday, 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," 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 U.S. 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".
Kerry stressed Washington's commitment to supporting anit-Islamic State fighters on the ground, the official said. The secretary of state also said that Assad's "brutality" against Syrians helped to incite foreign fighters to join Islamic State. Kerry said last month he planned to discuss with Lavrov how to combat Islamic State and the role Iran could play.
Shi'ite Muslim power Iran, an Assad ally, has also urged Gulf Arab states to work with it to counter Sunni militants like Islamic State in the region. Tehran, locked in a struggle with Riyadh for regional primacy, is viewed warily by most Gulf Arab states.