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Foreign Minister Sergey Lavrov of Russia, left, with Secretary of State John Kerry, right, at a meeting in Kuala Lumpur, Malaysia, in August.Credit Joshua Paul/Associated Press

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.

Russia has sent a military advance team to Syria and has transported prefabricated housing units for hundreds of people to an airfield near Latakia, according to American intelligence analysts. Russia has also delivered a portable air traffic station to the airfield and has filed military overflight requests through September.

While American officials have said they are not sure of Moscow’s intentions, they say the airfield could be used to transport military supplies for the Assad government or to carry out Russian airstrikes to help Syrian government troops. The housing could accommodate as many as 1,000 Russian military advisers and other personnel, according to American officials, and one official suggested that the eventual Russian deployment might be even larger. 

The United States-led coalition has been bombing Islamic State positions in Syria. The United States and Turkey are also conducting airstrikes to try to clear Islamic State militants from the northern border area and support moderate Syria opposition fighters there.

The State Department warning that a major Russian deployment might “risk confrontation,” officials said, pointed to the danger that Russian airstrikes might interfere with the air operations that the United States and its partners are carrying out in Syria, or the possibility that Russian aircraft or Russian-backed Syrian government forces might attack opposition groups Washington is supporting.

The Russian moves come as Mr. Kerry has sought to persuade Russia’s president, Vladimir V. Putin, to work with the United States on a political solution to the bloody conflict in Syria. The Obama administration has repeatedly said that any diplomatic solution must require that Mr. Assad, a Russian ally, relinquish power.

On Friday, Mr. Putin presented his own ideas, which he said had been endorsed by Mr. Assad. They called for holding “early elections” and bringing “healthy” elements of the opposition into a governing coalition.

Mr. Putin did not say which members of the Syrian opposition would be acceptable, but there have long been tensions between the Russian government and the moderate opposition backed by Washington.

Some analysts say there are different views in Moscow over whether Russia should strongly back Mr. Assad as a bulwark against the Islamic State or seek a political transition in which he might be eased out of power.

“The escalation is completely in keeping with hard-line Russian positions on Assad, which are juxtaposed with more conciliatory positions by the Ministry of Foreign Affairs,” said Andrew J. Tabler, an expert on Syria at the Washington Institute for Near East Policy. 

There was no immediate comment in Moscow on the State Department statement.