The US and Nato have expressed concern over reports that Russia is increasing its military presence in Syria.
Nato chief Jens Stoltenberg said if confirmed, Russia's involvement would not help to solve the conflict.
Separately on Wednesday, US Secretary of State John Kerry "reiterated" his concerns to Russian Foreign Minister Sergei Lavrov over the phone.
Russia, a key ally of Syria during its four-year civil war, says it has sent military experts but that is all.
Correspondents say that without Moscow's backing, Syrian President Bashar al-Assad may have fallen by now.
Unnamed US officials quoted by Reuters say Moscow has sent additional aircraft and two tank landing ships to Russia's naval base in the Syrian coastal city of Tartus, within the past day or so. They also say a small number of naval infantry forces have been deployed.
But a Russian foreign ministry spokeswoman insisted on Wednesday that this was nothing new, arguing that Moscow had been openly supplying weapons and sending military specialists to Syria for a long time.
"Russia has never made a secret of its military-technical cooperation with Syria,'' Maria Zakharova said. Syria, for its part, has denied any military build up by Russian troops on its soil.
Unnamed Lebanese officials have also told Reuters that Russian soldiers are already taking part in the fighting on the ground.
Meanwhile, a spokesman for the Russian embassy in Iran told Russian media that Moscow had been granted permission to fly planes over Iranian territory en route to Syria, but Iran has yet to confirm this. It follows Bulgaria's refusal to allow an unspecified number of Russian aircraft to cross over its air space.
John Kerry warned Mr Lavrov on Wednesday that if the reports were found to be true, "it could lead to greater violence". The German and French foreign ministers have also warned against a further military escalation.
The US is currently engaged in its own military air campaign against Islamic State (IS) militants in both Syria and Iraq.
US officials say Russia may be providing President Assad with greater military support because this year he has sustained substantial territorial losses, as IS militants have come to be seen as the bigger threat.
The latest developments follow reports of further setbacks for Syrian government forces, which on Wednesday lost control of a key air base in Idlib - one of the final regime position's in the north-eastern province.
Russia and the US disagree sharply on Syria. While Russia has backed the Syrian government, and provided it with arms, the US wants to see the removal of President Assad.
Syrian President Bashar al-Assad's regime has been at war with various rebel groups since 2011, in a conflict that has so far killed at least 240,000 people.