WASHINGTON — Secretary of State John Kerry told his Russian counterpart on Saturday that the United States was deeply concerned by reports that the Kremlin may be planning to vastly expand its military support for President Bashar al-Assad of Syria, warning that such a move might even lead to a “confrontation” with the American-led coalition, the State Department said.

Mr. Kerry called Russia’s foreign minister, Sergey V. Lavrov, “to discuss Syria, including U.S. concerns about reports suggesting an imminent enhanced Russian military buildup there,” the State Department said in an unusually blunt statement.

“The secretary made clear that 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 added, using an acronym for the Islamic State.

The statement did not say if Mr. Lavrov provided any sign of Russia’s intentions, but there was no indication that he had eased Mr. Kerry’s concerns. The statement noted that the two diplomats planned to continue their discussions in New York this month, when the United Nations General Assembly is to meet.