U.S. media reports said Russia has sent advisers and hardware to Syria, in what Washington fears is an expansion of its support for President Bashar al-Assad, while such development would "escalate the conflict", Kerry told his Russian counterpart Sergei Lavrov, according to BBC News.
As Russia has been a key ally of Syria throughout its four-year civil war, now it had dispatched an advance military team to Syria, as well as housing units and an air traffic control center to an airfield, The New York Times reported citing U.S. officials as saying.
The officials quoted admitted they were unsure of Moscow's intentions, but said the deployments could enable Russia to use the airbase to facilitate supply of military gear or to launch air strikes against Mr Assad's enemies.
John Kerry made his concerns over the reports known to the Russian Foreign Minister in a phone call.
"If such reports were accurate, these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-Isil coalition operating in Syria," the state department said, using an alternative acronym for Islamic State (IS).
Russian President Vladimir Putin said on Friday it was "premature" to talk about Moscow getting involved in direct fighting.
Russia and the U.S. disagree sharply on Syria. While Russia has backed the Syrian government, and provided it with arms, the US wants to see the removal of President Assad.