Russia has sent fighter jets to Syria, US officials said, raising the stakes in a military buildup that has put Washington on edge and led Friday to the first talks between US and Russian defense chiefs in over a year.

US Defense Secretary Ash Carter, eyeing the possibility of rival US and Russian air operations in Syria's limited airspace, agreed in a call with his Russian counterpart to explore ways to avoid accidental military interactions.

The coordination necessary to avoid such encounters is known in military parlance as "deconfliction."

"They agreed to further discuss mechanisms for deconfliction in Syria and the counter-ISIL campaign," Pentagon spokesman Peter Cook said after the call, referring to the campaign by the United States and its allies against Islamic State militants.

The former Cold War foes have a common adversary in Islamic State militants in Syria, even as 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.

A senior US defense official, recounting details of the conversation, said Russia's Defense Minister Sergei Shoigu had described Moscow's activities in Syria as defensive in nature.

Shoigu said Russia's military moves "were designed to honor commitments made to the Syrian government," the US official said.

It was unclear, however, what those commitments to Syria are or how Russia's military buildup was relevant to them.

Russia's latest deployment has added significant airpower to a buildup that, according to US estimates, also includes helicopter gunships, artillery and as many 500 Russian naval infantry forces at an airfield near Latakia.

One US official, speaking on condition of anonymity, said four tactical Russian fighter jets were sent to Syria. Another US official declined to offer a number but confirmed the presence of multiple jets.

In London, US Secretary of State John Kerry said the United States was looking to find "common ground" with Russia.

Kerry said it was important to forge a political agreement in Syria and end the hardship of Syrian people.

"Everybody is seized by the urgency. We have been all along but the migration levels and continued destruction, the danger of potential augmentation by any unilateral moves puts a high premium on diplomacy at this moment," he said.

Still, the White House cautioned Moscow against "doubling down on Assad."

Carter told Shoigu that future consultations would run in parallel "with diplomatic talks that would ensure a political transition in Syria," Cook said.

"He noted that defeating (Islamic State militants) and ensuring a political transition are objectives that need to be pursued at the same time," he said.

The last time a US defense chief spoke with Shoigu was in August 2014, the Pentagon said, adding high-level communications were halted following Russia's annexation of Crimea and its intervention in Ukraine.

Kiev and the West accuse Moscow of driving a pro-Russian separatist rebellion in east Ukraine, which started shortly after the Crimea annexation. Russia denies this.

Moscow's moves in Syria set the stage for Russian President Vladimir Putin's address to the United Nations General Assembly on Sept. 28, likely shifting some attention away from Ukraine and toward the conflict in Syria.

"The trajectory that Putin was on for UNGA was to come to New York and basically be ignored," said Andrew Weiss, vice president for studies at the Carnegie Endowment for International Peace.

"And now what he's done is ... put himself in the spotlight, and on an issue where there's a lot of tough questions for Western leaders about how they've been handling the crisis." – Reuters