Turkey says it's about to launch an air and ground military operation into northern Syria.
President Tayyip Erdogan accuses Washington of not doing enough to expel Syrian Kurdish fighters in the area from its border.
It will happen today or tomorrow, he says.
The U.S.-backed Syrian Democratic Forces have threatened an "all-out war" in response.
U.S. forces are stationed in the area too.
Turkish military reinforcements were sent to a border area on Saturday night (October 5), though Turkey hasn't confirmed they're connected to the planned incursion.
What Ankara wants is to create a zone along the border inside Syria and re-settle refugees there.
It wants the SDF, which is led by the Kurdish YPG militia, to clear out.
The SDF's spokesman said the force would not hesitate to turn what it called an unprovoked attack by Turkey into, quote, an "all-out war on the entire border to defend ourselves and our people".
The United States and Turkey agreed to set up the zone in August.
Now Turkey says Washington has been slow to act, and its patience has run out.
The NATO allies are also at odds over how far the zone should stretch into Syria and who should control it.
Ankara says it wants to settle up to two million refugees in the zone, about half the number of Syrians it shelters from the conflict.