U.S. Defense Secretary Mark Esper said on Tuesday any Turkish operation into northern Syria would be "unacceptable" and the United States would prevent unilateral incursions, as tensions between Washington and Ankara simmer.
On Sunday, President Tayyip Erdogan said Turkey, which already has a foothold in northwest Syria, will carry out a military operation in a Kurdish-controlled area east of the Euphrates in northern Syria.
"Clearly we believe any unilateral action by them (Turkey) would be unacceptable," Esper told reporters traveling with him to Japan.
"What we're going to do is prevent unilateral incursions that would upset, again, these mutual interests... the United States, Turkey and the SDF share with regard to northern Syria," Esper said.
The SDF stands for the Syrian Democratic Forces. With U.S. backing, the SDF, which includes the Kurdish YPG militia, have taken control over the last four years of much of northeastern Syria from Islamic State militants. Ankara sees the YPG as a terrorist organization.
Esper said the United States did not have any "ambition" to abandon the SDF, but stopped short of guaranteeing that the United States would protect them in case of a Turkish operation.
A team from the Pentagon was in Turkey to speak with Turkish officials about the issue and Esper said he was hopeful that an agreement could be reached with Ankara.
Esper suggested that a Turkish operation into northern Syria could affect the SDF's focus on ensuring Islamic State did not retake the territory it once held in Syria and the ability of the U.S.-backed forces to hold the thousands of alleged Islamic State fighters in detention.
The Turkish-led campaign, which has for months been delayed due to resistance from Washington, is aimed at evicting Kurdish YPG forces from a string of border town in Raqqa and Hasaka provinces.
Ankara has accused Washington of stalling progress on setting up a safe zone inside Syria's northeastern border with Turkey that would be cleared of the Kurdish YPG militia.
This week, Erdogan said both Russia and the United States had been told of the planner operation, but did not say when it would begin. It would mark the third Turkish incursion into Syria in as many years.
Ties between the two NATO allies have been strained over a host of issues, including the United States' removal of Turkey from the F-35 fighter jet program after Ankara bought and took delivery of Russian S-400 missile defenses that Washington sees as a threat.