Pentagon chief calls for talks to prevent unilateral incursion against YPG, as Erdogan vows imminent attack.
The United States has warned that any Turkish operation in northern Syria would be "unacceptable" and vowed to step up talks to prevent unilateral incursions, as tensions between Washington and Ankara simmer.
On Sunday, President Recep 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 against the US-backed Kurdish People's Protection Units (YPG) that Ankara views as a terrorist group.
"Clearly we believe any unilateral action by them [Turkey] would be unacceptable," US defence secretary Mark Esper told reporters in Tokyo on a trip through Asia on Tuesday.
"And so what we are trying to do now is work out with them an arrangement to address their concerns and I'm hopeful we'll get there," said Esper.