The negotiations follow the announcement by Turkish President Recep Tayyip Erdogan on Sunday that Turkey would launch a military operation against Kurdish-led autonomous administrations and militias east of the River Euphrates after previous offensives in 2016 and last year.

A Turkish military operation in northeast Syria would be unacceptable and undermine efforts to secure the Turkish-Syrian border and fight against the remnants of Islamic State (IS), U.S. State Department spokeswoman Morgan Ortagus said.

“We entered Afrin, Jarablus, and Al-Bab. Now we will enter the east of the Euphrates,” the Turkish president said, referring to towns captured in the previous Turkish offensives.

This would be unacceptable for the United States, Ortagus said, urging Turkey to continue working with Washington to resolve the situation.

“We would find any such actions unacceptable and thus urge Turkey once again to work with us on a joint approach,” she said.

The possibility of a Turkish operation has sharply increased since July, when tens of thousands of Turkish troops began massing at the southern border. Statements from the defense and foreign ministers have signaled that Ankara has lost patience with U.S.-Turkish negotiations to prevent a Turkish assault.

Ankara views the Syrian Kurdish groups controlling the area as extensions of the Kurdistan Workers’ Party (PKK), which is listed as a terrorist organization by both Turkey and the United States.

But U.S. forces have been fighting IS alongside these groups, the Syrian Kurdish People’s Protection Units (YPG), which forms part of the multi-ethnic Syrian Democratic Forces (SDF), and Washington firmly opposes a Turkish operation against them.

U.S. efforts to prevent a conflict by negotiating a safe zone along the border in northern Syria have stalled due to disagreements over the extent and administration of the safe zone, but Ortagus said the safe zone was still the U.S. administration’s favored solution.

“We continue intensive discussions with Turkey on a safe zone to address their legitimate security concerns along the Turkey-Syria border. We believe this dialogue is the only way to secure the border area in a sustainable manner,” Ortagus said in an emailed response to questions, Ahval news agency reported.

“Any uncoordinated military operations by Turkey will undermine that shared interest. Such unilateral military action into northeast Syria, particularly as U.S. personnel may be present or in the vicinity, while our and local Syrian partner’s operations against IS holdouts are continuing, is of grave concern,” she said.

Reports from northern Syria describe an upsurge of IS activity since tensions flared on the border, with jihadists setting off three bombs in the countryside of the northeastern Al-Hasakah province.

Kurdish commanders have said they are prepared for a Turkish onslaught, but that any battles on the border would draw troops away from the fight against IS, risking a resurgence of the extremist jihadist group.

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