United States and Russian defence ministers have held their first talks in more than a year to discuss the conflict in Syria.
A rebel fighter fires heavy artillery against government forces in Syria's Idlib province. Photo: AFP
The phone call followed signs that Moscow was taking a more active role in the conflict and American concern over the extent of Russia's plans.
Meanwhile, four Russian fighter jets have arrived at an airfield near the Syrian city of Latakia, the US said.
The latest deployment has added significant airpower to the helicopter gunships, artillery and as many as 500 naval infantry forces the Russians have sent to help the Assad regime.
The former Cold War foes have a common adversary in Islamic State fighters in Syria, but Washington opposes Moscow's support for Syrian President Bashar al-Assad, seeing him as a driver in the nation's devastating four-and-a-half-year civil war.
While Moscow has backed the Syrian government, the US sees the removal of President Assad as essential to resolving the conflict.
Red Crescent workers unload Russian humanitarian aid amid reports Moscow is beefing up military support to Damascus. Photo: AFP / HO / SANA
Secretary of Defence Ashton Carter discussed with Russia's Defence Minister Sergei Shoigu how their two sides could avoid accidentally clashing on the ground, the Pentagon said.
Russian state media said the talks proved the sides had common ground.
The role of Mr Assad was a key point of difference between Moscow and Washington.
The US has been alarmed about reports of a Russian military build-up in Syria at a time when the Assad government has been losing ground to rebels.
Both Russia and the US were concerned about the rise of the Islamic State militant group, which now controlled parts of northern Syria.