Blinken urged Turkey to stick to cease-fire lines established in 2019 after President Recep Tayyip Erdogan renewed threats to "clean up" two northern Syrian cities of Kurdish fighters.

"It's something that we would oppose," Blinken told a joint news conference with NATO Secretary-General Jens Stoltenberg.

"The concern that we have is that any new offensive would undermine regional stability (and) provide malign actors with opportunities to exploit instability," Blinken said.

The United States has partnered with Syrian Kurdish fighters to fight the Islamic State movement, also known as ISIS or Daesh, in war-battered Syria.

But Turkey considers the Syrian Kurdish fighters part of the PKK, separatists considered terrorists by Ankara.

"We continue effectively to take the fight through partners to Daesh -- to ISIS -- within Syria and we don't want to see anything that jeopardizes the efforts that are made to continue to keep ISIS in the box that we put it in," Blinken said.