"It has detracted from our fight to defeat Islamic State in eastern Syria," Tillerson said of ongoing offensives by the Kurdish forces to clear remaining pockets of extremists.

"Forces have diverted from there to Afrin," he said.

Turkey has repeatedly called on the United States to stop working with the Kurdish YPG.

On Sunday, Ankara threatened to expand its anti-YPG operation east of the Euphrates, warning that American soldiers risked being targeted in the area if they wore enemy uniform.

Tillerson said he would be in Ankara later in the week to determine a path forward with Turkish counterparts.

"We have been in discussions with our NATO ally Turkey," he told reporters in Kuwait City, adding that "We think it's important that they are mindful of the effect is having on our principal mission, which is the defeat of IS."

Reporter’s code: 50101