The White House said it is closely monitoring reports that Russia is carrying out military operations in Syria and warned such operations could be "destabilizing and counter-productive."
"We are aware of reports that Russia may have deployed military personnel and aircraft to Syria, and we are monitoring those reports quite closely," said spokesman Josh Earnest.
"Any military support to the Assad regime for any purpose, whether it's in the form of military personnel, aircraft supplies, weapons, or funding, is both destabilizing and counterproductive."
The comments come after images appeared on a social media account linked to Syrian fighters purporting to show Russian aircraft and drones near Idlib province.
Unconfirmed reports suggested the aircraft may have included a Russian Sukhoi 34 advanced strike fighter, which Syria is not thought to own.
A U.S. official confirmed that "Russia has asked for clearances for military flight to Syria," but added "we don't know what their goals are."
"Evidence has been inconclusive so far as to what this activity is."
Other reports have suggested Russia has targeted Islamic State militants, who have attacked forces loyal to Russian-backed Syrian leader Bashar al-Assad.
Both the White House and the Pentagon refused to say whether they had intelligence suggesting the reports were accurate.
"It's up to the Russians to explain exactly what they are doing," said Peter Cook, Pentagon spokesman.
The United States and its allies have repeatedly called for Assad's ouster, and are likely to oppose any effort that would have the effect of supporting him.
The White House said, however, that it would welcome Russia's involvement in an international coalition that is battling IS in Iraq and Syria.
The Kremlin said September 2 that it does not plan to participate in the campaign against IS. But it confirmed September 3 that it has been providing military assistance to Syria.
"You know that there is interaction between the Russian Federation and Syria. Cooperation is being maintained," Russian presidential press secretary Dmitry Peskov said when asked whether Moscow is stepping up its military assistance to Damascus.
"This is a permanent process. I wouldn't like to give you more specific details," he said.