Members of the Kurdish People's Protection Units, or YPG, attend a funeral Saturday for Kurdish fighters from the Syrian Democratic Forces who were killed in combat against the Islamic State in northeastern Syria. (Delil Souleiman/AFP/Getty Images)

U.S.-allied forces in eastern Syria said Tuesday that they are withdrawing from the front lines of the war against the Islamic State in order to battle the United States’ NATO ally Turkey elsewhere in the country, jeopardizing the fight against the militants.

Citing disappointment with the United States, the Kurdish-dominated Syrian Democratic Forces (SDF) said they were pulling fighters off the front lines in the province of Deir al-Zour, where Islamic State fighters have been putting up a fierce fight in a pocket of territory on the eastern bank of the Euphrates River. The holdouts there are thought to include some of the most senior leaders of the organization who escaped the cities of Mosul in Iraq and Raqqa in Syria last year, U.S. officials say.

The move follows an effort by the Trump administration to assuage Turkish ire over the U.S. military’s close relationship with Syrian Kurdish forces. Those forces have been instrumental in the capture of vast swaths of territory from the Islamic State over the past three years, including the militants’ self-styled capital, Raqqa.

According to a statement by the SDF, which has received arms and training from the United States, the fighters will relocate to the Kurdish-controlled enclave of ­Afrin in northwestern Syria to help fend off a six-week-old Turkish offensive that has reportedly killed hundreds of civilians, displaced more than 10,000 people and contributed to one of the worst crises in U.S.-Turkish relations in decades.

It is with “regret” that the “painful decision” has been made to pull fighters away from the battle in Deir al-Zour, the SDF statement said. The decision would not have been taken, it added, “were it not for the failure of the international community to curb the Turkish aggression and put real pressure” on the government of President Recep Tayyip Erdogan to “stop its madness within our Syrian borders.”

The SDF withdrawal was a direct result of Kurdish frustration with the recent American rapprochement with Turkey and the lack of support Washington has given to the Kurds in their fight against Turkish troops in Afrin, according to Aldar Xelil, a senior official with the self-proclaimed Kurdish-led administration governing northeastern Syria.

“The international coalition let us down,” he said in an interview over Skype. “They did not do what we expected them to do for us after a very long partnership.”

“We are allies. The Americans should have helped us. We were allies for a very long time,” he added. “For one and a half months we have been under attack by Turkey. Turkey is using NATO weapons to attack an American ally. We were partners in the fight against [the Islamic State], and they did not do anything to help us.”

The redeployment involves about 1,500 to 1,700 fighters drawn mostly from Arab groups affiliated with the Kurdish-
dominated SDF. Thousands of Kurdish fighters had previously left northeastern Syria for Afrin, and operations against the Islamic State had already come to a standstill, Xelil said. Some SDF fighters will remain on the front line there, alongside U.S. troops, and will continue to defend their positions, he said.

In recent months, the Islamic State fighters have begun to regroup, Xelil warned. “There is a danger this will give ISIS a chance to revive, to come back to life, and they might even expand their territory again,” he said.

The U.S. military said operations against the Islamic State had slowed but not stopped entirely, and warned that U.S. support for the SDF could be jeopardized by the withdrawal from the fight.

“We are aware of the departure of some SDF forces from the Middle Euphrates River Valley and continue to point out the potential costs of any distraction from the defeat-ISIS fight,” Maj. Adrian Rankine-Galloway, a U.S. military spokesman, said in an emailed statement. “The Coalition will achieve its goals, but the increased complexity of the situation in Syria can result in operations taking longer.” U.S. support for the SDF will continue “as long as they remain focused on the defeat-ISIS fight,” Rankine-Galloway added. “Any military efforts outside those specifically focused on defeating ISIS do not, and will not, receive coalition support.”

The SDF withdrawal reflects the deepening complexities of the U.S. involvement in Syria as the fight against the Islamic State winds down, leaving about 2,000 U.S. troops in de facto control of a vast swath of northeastern Syria alongside a Kurdish-dominated force that is anathema to Turkey.

The Syrian Kurdish group that dominates the SDF is closely affiliated with the Kurdistan Workers’ Party, or PKK, which is designated a terrorist organization by Turkey and the United States. Over the past three years, Turkey has repeatedly expressed its frustration over the level of U.S. military support for the Kurdish People’s Protection Units, or YPG, which is the main component of the SDF and has been described in U.S. intelligence reports as the Syrian branch of the PKK.

A visit by U.S. Secretary of State Rex Tillerson to Ankara last month went a long way toward assuaging Turkish concerns. Tillerson described Turkey as the “enduring” ally in the complicated triangular relationship, and a meeting is due to take place in Washington on Thursday to establish a mechanism for what U.S. officials have described as “diluting” the YPG’s level of control in northeastern Syria.

On the table is the fate of Manbij, a majority-Arab town in northern Syria that is under the control of the SDF despite promises to Turkey made by the Obama administration that the Kurdish allied forces would withdraw after the Islamic State was driven out. The SDF statement on the withdrawal from the Islamic State fight may have been timed ahead of the talks to put pressure on the United States not to surrender Manbij to Turkey, analysts said.

That the Trump administration has alienated the Kurds in seeking to regain Turkish trust reflects the complexities of a war that pushed the United States into alliances with multiple armed groups in Syria and Iraq that are often at odds with one another.

The Turkish-Kurdish dispute predates the Islamic State war by decades and has now left the United States in “an impossible position,” said Aaron Stein of the Atlantic Council. “The U.S. is trying to manage a war in Syria while dealing with two allies who can’t stand each other,” Stein said. “The U.S. refuses to admit it has a major problem here.”

The Trump administration is divided over whether to prioritize repairing the U.S. alliance with Turkey or to remain loyal to the Kurds. The U.S. military favors the Kurdish allies it has fought alongside, and the State Department is mindful of Turkey’s broader importance to the United States’ NATO relationships and the building tensions with Russia, Stein added. “The U.S. is stuck, and there are really no good outcomes here,” he said.

Zakaria Zakaria in Istanbul and Heba Habib in Stockholm contributed to this report.