Secretary of State John Kerry on Saturday night expressed the United States’ concern over reports of Russia's enhanced military build-up in Syria in a telephone call with his Russian counterpart, the State Department said, according to Reuters.
"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 department said, using an acronym for Islamic State.
It added that Kerry and Russian Foreign Minister Sergei Lavrov agreed that discussions on the Syrian conflict would continue this month in New York, where the UN General Assembly meets.
Media reports on Friday quoted U.S. officials as describing an increase in Russian military activity in Syria, expanding Moscow's support for Syrian President Bashar Al-Assad.
Quoting unidentified Obama administration officials, the New York Times reported that Russia has dispatched a military advance team to Syria and has sent prefabricated housing units for hundreds of people to a Syrian airfield and delivered a portable air traffic control station there.
Earlier on Saturday, new images came to light which appear to confirm previous reports of Russian "boots on the ground" in Syria.
The pictures appear to be taken in a variety of locations in western Syria, including Homs and the key port city of Tartus. They show Russian fighters posing with weapons in front of Russian and Syria flags, and portraits of the countries' respective leaders Vladimir Putin and Assad.
Lavrov said last month the United States should cooperate with Assad, a longtime Moscow ally, to fight Islamic State forces who have seized parts of northern and eastern Syria.
Russia has protected its ally in the UN, having vetoed several Security Council resolutions which sought to criticize Assad.
The Security Council recently adopted a new push for peace talks in Syria, which was also endorsed by Russia and the other 14 member states.
The peace initiative, set to begin this month, would set up four working groups to address safety and protection, counterterrorism, political and legal issues and reconstruction.