US Secretary of State John Kerry has raised US concerns about reports of "an imminent enhanced Russian military buildup" in Syria, in a phone call to his counterpart in Moscow, the US State Department said.
"The secretary made clear that 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 on Saturday.
Kerry spoke with Russian Foreign Minister Sergei Lavrov, discussing with him the reports of Russia's ambitions to boost its military intervention in the Arab country.
The New York Times reported on Friday that Russia had sent a military advance team to Syria and was taking other steps that Washington fears may signal plans to vastly expand its military support for President Bashar al-Assad.
The paper said the moves included the recent transport of prefabricated housing units for hundreds of people to a Syrian airfield and the delivery of a portable air traffic control station there.
Related: US scolds Russia for sending arms to Syria
Russia's President Vladimir Putin was asked Friday whether Russia was taking part in military operations against the Islamic State of Iraq and the Levant group in Syria.
"We are looking at various options but so far what you are talking about is not on the agenda," he said.
"To say we're ready to do this today - so far it's premature to talk about this. But we are already giving Syria quite serious help with equipment and training soldiers, with our weapons," RIA Novosti state news agency quoted Putin as saying.
It made no mention of US concerns about a possible Russian military buildup, but said the two discussed "different aspects of the situation in Syria and its environs, as well as the objectives of the fight against ISIL and other terrorist groups".
Moscow has never voiced support for moderate Syrian rebels and has referred to them as "terrorists" because of their opposition to Assad forces.
The US fears Russia's alleged military buildup in Syria would counter Syrian groups who are battling both ISIL and government troops.