Secretary of State John Kerry delivers a speech at the National Constitution Center, Wednesday, Sept. 2, 2015, in Philadelphia.(Photo: Matt Slocum, AP)

WASHINGTON — Further Russian intervention in Syria’s civil war could make things worse for civilians and the coalition battling Islamic State fighters there, Secretary of State John Kerry warned Russian Foreign Minister Sergey Lavrov in a phone call Saturday.

Kerry made the call in response to reports “suggesting an imminent enhanced Russian military build-up” in Syria, according to readout of the call from the State Department.

“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 statement said.

Syrians have been fleeing a four-year civil war as well as advances by Islamic State militants, who've taken over large swaths of the country. The refugees have turned up in the tens of thousands this summer in Europe and elsewhere in the region. Millions have sought refuge in nearby Turkey, Lebanon and Jordan.

Reports that U.S. intelligence officials have found evidence of a growing Russian military presence has raised concerns at the White House that Russian President Vladimir Putin is lending even more assistance to Syrian President Bashar Assad than previously known. Putin has been providing financial support and intelligence to the Assad regime for years.

“I will just say as a general matter that any military support to the Assad regime for any purpose, whether it's in the form of military personnel, aircraft, supplies, weapons, or funding, is both destabilizing and counterproductive,” White House spokesman Josh Earnest said Thursday.

Citing unnamed administration officials, the New York Times reported Saturday that Russia recently moved prefabricated housing units and a portable air traffic control station to a Syrian airfield. The movements could be preparations for about 1,000 military advisers or personnel, but there was no confirmation that thousands of ground troops were on the way, the paper reported.

U.S. officials have not confirmed Moscow’s intentions but they are concerned the aid could be used against U.S.-backed opponents of Assad’s government, the Los Angeles Times reported.

Sen. Lindsey Graham of South Carolina, a Republican candidate for president, called the developments “a slap in the face to President Obama and John Kerry.”

“This shows you how little respect Russia has for us,” Graham said Saturday. “This strategically significant movement, if true, ensures Assad is more likely to stay in power, which prolongs the conflict and makes an attack on our homeland even more likely.”

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