Russia will consider sending troops to Syria if it receives a request from Damascus, Kremlin spokesman Dmitry Peskov said on Friday.
"If there is a request, then in the framework of bilateral contracts, it will be discussed and considered. For now, it is rather difficult to speak hypothetically," Peskov was quoted by Xinhua as saying.
Russia has long been supplying weapons and military equipment to Syria, which Moscow said "is done under the existing bilateral contracts and in full conformity with international law," and is aimed at "ensuring Syria's defence capabilities" to fight against the Islamic State and other terrorist groups in the region.
Moscow has also sent soldiers and military experts to service the military equipment and teach Syrian servicemen to use it, but it has ruled out their participation in combat operations so far.
US Secretary of State John Kerry earlier told Russian Foreign Minister Sergei Lavrov about Washington's concerns over reports of alleged Russia's military build-up in Syria.
The Russian Foreign Ministry said on Thursday that Moscow was prepared to provide Washington with information on its military cooperation with Damascus.
Syrian Foreign Minister Walid al-Moallem said late on Thursday that there are no Russian troops fighting alongside the Syrian army in the war-torn country, and the "Russian presence in Syria is so far confined to military experts and specialists."