"Clearly we believe any unilateral action by them would be unacceptable," Defense Secretary Mark Esper told reporters in Tokyo 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.

Washington has supported the YPG as the main fighting force against the Islamic State group in Syria.

But Ankara sees it as an offshoot of the Kurdish PKK, which has fought a bloody separatist insurgency inside Turkey for the past 35 years.

Turkey has so far been unimpressed with the details of the 30-kilometre (18-mile) US "safe zone" plan in Syria and has renewed threats to launch a cross-border offensive if the talks fail to reach a "satisfactory" conclusion.

"We can only be patient for so long. That patience will come to an end," Erdogan said on Sunday.

Turkish media has regularly shown images in recent weeks of military convoys heading for the border area, carrying equipment and fighting units.

Turkey has twice carried out unilateral offensives into northern Syria against the Islamic State group and YPG, in 2016 and 2018 respectively.

Aldar Khalil, a top Kurdish political official in northeast Syria, told AFP Monday that "Erdogan is serious and will embark on an attack at the first opportunity" against Kurdish fighters.