US Secretary of State John Kerry has expressed concern to Moscow over reports of Russian military build-up in Syria, the state department says.
US media reports said Russia has sent advisers and hardware to Syria, in what Washington fears is an expansion of its support for President Bashar al-Assad.
Any such development would “escalate the conflict”, Mr Kerry told his Russian counterpart Sergei Lavrov.
Russia has been a key ally of Syria throughout its four-year civil war.
The New York Times cited US officials as saying Russia had despatched an advance military team to Syria, as well as housing units and an air traffic control centre to an airfield.
The officials quoted admitted they were unsure of Moscow’s intentions, but said the deployments could enable Russia to use the airbase to facilitate supply of military gear or to launch air strikes against Mr Assad’s enemies.
Image copyright AFP Image caption A 2011 uprising against President Assad has transformed into a complex civil war
Mr Kerry made his concerns over the reports known to the Russian Foreign Minister in a phone call.
“If such reports were accurate, these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-Isil coalition operating in Syria,” the state department said, using an alternative acronym for Islamic State (IS).
Russian President Vladimir Putin said on Friday it was “premature” to talk about Moscow getting involved in direct fighting.
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.
In the latest fighting on the ground, at least 47 people died in fierce clashes between IS militants and other rebels in the northern town of Marea.
At least six members of Syria’s security forces were shot dead in the south-western city of Sweida, after a prominent cleric in the minority Druze community died in a car bomb blast.