Russian defense chiefs spoke for the first time in over a year on Friday, breaking their silence to discuss the crisis in Syria as Moscow's increasing military buildup there raised the prospect of coordination between the former Cold War foes.

The Pentagon said the call lasted about 50 minutes and included an agreement for further US - Russian talks about ways to keep their respective militaries out of each other's way, something known as "deconfliction" in military parlance.

The United States fiercely opposes Russia's support for Syrian President Bashar al-Assad. The Pentagon last year cut off high-level military talks with Moscow after Russia's annexation of Crimea and intervention in Ukraine. But Russia's buildup at Syria's Latakia airbase raises the possibility of simultaneous US and Russian air combat missions in Syrian airspace.

Heavy Russian equipment such as tanks and helicopters, as well as naval infantry forces, have recently been moved to Latakia, US officials say. US Defense Secretary Ash Carter has told Russian counterpart Defense Minister Sergei Shoigu that future  conversations would run in parallel "with diplomatic talks that would ensure a political transition in Syria," the Pentagon said.

"He noted that defeating (ISIL terrorist group) and ensuring a political transition are objectives that need to be pursued at the same time," the Pentagon said. The last time a US defense chief spoke with Shoigu was in August 2014, the Pentagon said.

oth Moscow and Washington say their enemy is ISIL terrorist group, whose fighters control parts of Syria. Washington believes that Assad's presence is fueling the Syrian crisis, but Russia continues to support its long-time ally.

In London, US Secretary of State John Kerry said the United States was looking to "find a common ground" with Russia. Kerry said it was important to forge a political agreement in Syria and end the hardship of Syrian people;" Reuters reported.