Turkey has carried out air strikes against semi-autonomous Kurdish zones in Syria and Iraq since a deadly Istanbul bombing it blames on Kurdish groups, and has threatened to launch an operation on the ground in Syria.

The US-supported Syrian Democratic Forces (SDF), now the Kurds' de facto army in northeast Syria, have played a key role in dislodging IS fighters from the territory they seized in the country.

"The continued conflict, especially a ground invasion, would severely jeopardize the hard-fought gains that the world has achieved against ISIS and would destabilize the region," Pentagon Press Secretary Brigadier General Pat Ryder told journalists, using an acronym for IS.

"We... remain concerned about a potential Turkish ground operation in Syria, and again would urge restraint," he said, while also acknowledging Ankara's security concerns.

Ryder said US forces have reduced the number of joint patrols with the SDF, but have not redeployed.

"We have reduced the number of patrols because... we do these partnering with the SDF, and so they have reduced the number of patrols that they're doing," he said.

Since 2016, Turkey has launched several incursions against Kurdish forces in northern Syria that have allowed it to control areas along the border.