The proposed arrangement, including withdrawal from the zone of Syrian Kurds, who have been crucial U.S. allies in the fight against the Islamic State, marks a step back from initial Trump administration hopes that coalition allies or local security would secure the area.

The patrols would be an additional task for U.S. forces in Syria, whose numbers are due to be cut by more than half, to about 1,000, in the coming months. Britain and France, whose forces continue to participate in the U.S.-led counterterrorism mission against Islamic State remnants, have rejected an American request to contribute to what will be a buffer between the Kurds and Turkey.

Ankara considers the Kurds to be terrorists.

The border issue is one of several conflicts that have seriously disrupted the U.S.-Turkish relationship and put the two NATO allies on a collision course. Despite a steady stream of high-level Turkish officials visiting Washington in recent days, there has been no apparent progress in resolving U.S. demands that Turkey cancel its order for a Russian missile defense system, or risk being cut off as a purchaser and a participant in the United States' F-35 Joint Strike Fighter jet program.

Turkey has said its purchase of Russia's S-400 system is a "done deal."

Alexander Mikheev, the head of the Russian state arms exporter, Rosoboronexport, said Wednesday that "everything has been already discussed and agreed" upon and that delivery is planned to begin in July.

Turkish President Recep Tayyip Erdogan has rejected U.S. and NATO insistence that "co-location" of the Russian system and the F-35, the fifth generation of U.S. jet fighters, is unacceptable. Congress has threatened sanctions against Turkey if it goes ahead with the deal.

Reporter’s code: 50101