"Clearly we believe any unilateral action by them would be unacceptable," Defense Secretary Mark Esper told reporters on a trip through Asia.

"And so what we are trying to do now is work out with them an arrangement to address their concerns and I am hopeful we will get there... what we are trying to do is prevent unilateral incursions," said Esper.

Esper's comments came two days after Turkish President Recep Tayyip Erdogan vowed to launch an operation in northern Syria east of the Euphrates River against the US-backed Kurdish YPG militia, which Ankara views as a terrorist group.

Last-ditch talks are ongoing between US defence officials and their Turkish counterparts aimed at creating a "safe zone" in northern Syria to keep the YPG away from Turkey's border.