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.

“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 that the United States, Turkey and the SDF share with regard to northern Syria,” Esper said.

Esper said the United States did not have any “ambition” to abandon the SDF.

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 US-backed forces to hold the thousands of alleged Islamic State fighters in detention.

The Pentagon Chief’s comments came after Turkish President Recep Tayyip Erdoğan has repeatedly vowed to launch a comprehensive operation against northern Syria, amid the already ongoing intensified attacks against the region during the past weeks.