TOKYO: US defence secretary Mark Esper Tuesday said 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 travelling with him to Japan. “What we’re going to do is prevent unilateral incursions that would upset, again, these mutual interests… the US, Turkey and the SDF share with regard to northern Syria,” Esper said.

The SDF stands for the Syrian Democratic Forces. With US backing, the SDF, which includes the Kurdish YPG militia, have taken control over the last four years of much of northeastern Syria from Daesh. Ankara sees the YPG as a terrorist organisation. Esper said the US did not have any “ambition” to abandon the SDF, but stopped short of guaranteeing that the US would protect them in case of a Turkish operation.

A team from the Pentagon is in Turkey to speak with Turkish officials about the issue. Esper suggested that a Turkish operation into northern Syria could affect the SDF’s focus on ensuring Daesh did not retake the territory it once held in Syria.