If such a buildup occurred, it "could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-ISIS Coalition operating in Syria," the State Department said in a written statement.

Kerry and Lavrov agreed to continue their discussion of Syria in New York later this month, the statement said.

The Los Angeles Times, citing unnamed United States officials, reported that U.S. intelligence has captured evidence Russia is increasing its military presence in Syria's civil war.

The evidence reportedly includes satellite images of what looks like a base under construction that could be used for staging troops and heavy equipment, the officials told the newspaper.

Elise Labott, CNN's global affairs correspondent, said Russia and the United States had been talking about finding a political solution in Syria, which is gripped by a deadly civil war and threatened by ISIS.

"If the Russians are indeed increasing their military presence in Syria, that would certainly complicate that," Labott said.

She said Russia might be escalating air strikes against ISIS but that the real concern is that the Russians are increasing their activity on behalf of Syrian President Bashar al-Assad against the opposition. Or, said Labott, the Russians "could be taking steps to secure their own position if the Assad government falls."