(Bloomberg) — Defense Secretary Mark Esper told his Turkish counterpart that the U.S. opposes Turkey’s “uncoordinated actions” in Syria and that the country risks “serious consequences” if its military continues to operate there.

Esper, in a call with Turkish Minister of National Defense Hulusi Akar on Thursday, also said Turkey’s operations in Syria risk progress made in recent years by the coalition to defeat Islamic State and warned against any moves that could harm U.S. personnel in the region.

“While the Secretary reaffirmed we value our strategic bilateral relationship, this incursion risks serious consequences for Turkey,” Esper said in the call, according to a Pentagon statement published Friday. “Secretary Esper strongly encouraged Turkey to discontinue actions in northeastern Syria in order to increase the possibility that the United States, Turkey and our partners could find a common way to deescalate the situation before it becomes irreparable.”

The statement marks the latest effort by the Trump administration to argue that the president’s decision on Sunday to pull forces back from key posts in northern Syria didn’t constitute a green light for Turkey — a NATO ally — to send forces across the border and attack Kurdish-led forces long allied with the U.S. in fighting Islamic State.

Defending his decision against a wave of bipartisan criticism, President Donald Trump says he’s carrying out campaign promises to extract the U.S. from “endless wars,” but critics including some of his strongest Republican supporters in Congress say he’s exposed loyal allies to the risk of slaughter.

“I don’t think the American people want to see us go back in with our military, go back into that area again,” Trump told reporters in Washington on Thursday after tweeting about the options as he saw them. “We won, we left the area, I don’t think we want to go back in. Let’s see what happens.”

Earlier Friday, Akar said 342 fighters of the Kurdish militant group YPG have been killed since the operation began, a claim that was impossible to verify independently. At least two Turkish soldiers were also killed.

Mortar attacks by the YPG from areas under its control in Syria have killed nine civilians in southeast Turkey, including a 9-month-old baby of a Syrian refugee family, state-run TRT television said.