The United States and Turkey are negotiating a plan for their troops to jointly patrol a safe zone about 20 miles wide along Syria’s northeastern border with Turkey, according to officials from both countries.

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” 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.

Tensions increased this week with the Trump administration’s announcement that it would no longer waive sanctions against foreign companies and financial institutions in countries that depend on Iranian oil. Turkey, with a pipeline across their shared border, is a primary Iranian customer and stands to take a significant economic hit if the flow is shut off.

Ankara has protested the “secondary” sanctions, due to take effect May 2, as an illegal violation of its sovereign right to trade with any country it wishes. It has asked for more time to reduce its dependence on Iranian oil, but the administration’s position remains that there will be no waiver extensions.

Last month, senior Trump administration officials expressed hope that relations would thaw following Turkey’s municipal elections in late March. But despite a steady stream of high-level diplomatic, military and trade talks over the past several weeks, only the border issue appears to show some glimmer of progress.

“There are still differences of significance, but will on both sides to find solutions,” said a senior U.S. government official, who said the administration would like a narrower strip of land than the approximately 20 miles the Turks have proposed. The official spoke on the condition of anonymity because negotiations are ongoing.

Trained and armed by the United States, the Syrian Kurds of the People’s Protection Units — YPG by its Kurdish initials — have been the primary ground forces in the U.S.-led coalition’s fight against the Islamic State in Syria. The militia is the military arm of the Democratic Union Party, the main Kurdish political organization in Syria.

Turkey has charged that they are terrorists allied with its own Kurdish separatist movement and called on the United States to disarm and demobilize them. The United States, which has said that thousands of Islamic State fighters remain dispersed or underground in Syria, has refused.

Last year, as the Trump administration claimed victory in ejecting the Islamic State from Syrian territory it occupied along the Euphrates River, Erdogan massed troops on the border and threatened an offensive against the Kurds if they were not removed. That threat gained additional heft when President Trump, during a mid-December telephone conversation with Erdogan, said he was planning to withdraw about 2,000 U.S. troops in Syria.

Trump also agreed to a protected zone that would keep the Kurds away from the Turkish border. But the immediate question was who would protect it.

Turkey said it would send in its own troops — along with resettled Syrian refugees in Turkey who had fled violence in Syria — and asked for U.S. logistics and air cover. The administration, eager to avoid a situation that would in effect move the border 20 miles south, into Syria, and not remove the threat of an armed clash with the Kurds, appealed for coalition allies to position themselves as a buffer.

They refused, saying their mission was only to fight the Islamic State. A subsequent Trump administration plan to train non-YPG Syrians as a protection force never fully materialized.

In addition to ongoing discussions over the width of the zone to be patrolled, YPG leaders said last week that they had not yet been informed of U.S. withdrawal plans.