US and Russian defence ministers have held their first talks in more than a year, to discuss the conflict in Syria.
The phone call follows signs that Moscow is taking a more active role in the conflict, and American concern over the extent of the plans.
Meanwhile, four Russian fighter jets have arrived at an airfield near the Syrian city of Latakia, the US said.
The US and Russia have disagreed sharply on Syria's bloody civil war and the role of President Bashar al-Assad.
While Moscow has backed the Syrian government, the US sees the removal of President Assad as essential to resolving the conflict.
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 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.
The deployment of four jets at Latakia boosts Russia's military presence, which already includes helicopter gunships, artillery and as many as 500 Russian naval infantry forces, US officials said on Friday.
Both Russia and the US are concerned about the rise of the Islamic State militant group, which now controls parts of northern Syria.
Analysis: Jonathan Marcus, BBC defence correspondent
The growing Russian military presence in Syria, not least the deployment of surface-to-air missiles to defend the airfield at Latakia, means that Washington and Moscow have a lot to talk about.
The phone call between Mr Carter and Mr Shoigu is only the first step.
The US and a number of its allies are flying strike missions into Syrian air space and they do not want to have any misunderstandings with Russia's forces there.
The Americans also want to get a clearer idea as to the purpose of the Russian presence in Syria.
Is this simply to secure a bridgehead to re-supply Mr Assad? Or does it herald a Russian intervention in the fighting?
Military talks between Moscow and Washington could also facilitate a better understanding on the diplomatic front with Syria likely to be a prominent issue in the crucial contacts on the margins of the UN General Assembly later this month.
News of the phone call between Mr Carter and Mr Shoigu emerged shortly after Secretary of State John Kerry said the US hoped military-to-military conversations would take place "very shortly".
The defence chiefs discussed areas where "perspectives overlap and areas of divergence," the Pentagon statement said, describing the talks as "constructive".
In the 50-minute conversation, Mr Shoigu told Mr Carter that Russian activities in Syria were "defensive in nature," a US official told Reuters.
The two sides also agreed to further talks, reopening formal contact after relations were badly strained by Russian action last year in Ukraine.
The last conversation between the US defence chief and his Russian counterpart was last August when Mr Carter's predecessor Chuck Hagel held the office.
Meanwhile, Moscow has said that any request from Syria to send troops would be "discussed and considered".
A Russian human rights body said it has been contacted by Russian soldiers who fear being sent to fight in Syria.
Any secret deployment of troops to Syria would be illegal, said Sergei Krivenko of the Russian Human Rights Council.