(Bloomberg) -- The U.S. said it will stand aside when Turkey’s military launches an operation against America’s wartime Kurdish allies in Syria, a significant shift in American policy that raises questions over the fate of thousands of Islamic State detainees.

The Kurdish-led Syrian Democratic Forces have been a close U.S. ally in the fight to defeat Islamic State. But Turkey considers Syria’s Kurdish militants a threat to its national security and President Recep Tayyip Erdogan has said his forces were ready to begin a military operation against them in northeastern Syria imminently.

The decision represents a dramatic reversal for U.S. policy, which in 2015 provided air support for Kurdish militias to retake the critical town of Kobani from Islamic State and has since used Kurdish fighters as ground troops in the campaign to clear Syria of the group.

The shift could cast further doubt on the reliability of the U.S. as an ally in the region, in the wake of its abandonment of strongmen such as Egyptian President Hosni Mubarak during the 2011 Arab Spring uprisings.

As recently as January, U.S. President Donald Trump had warned Turkey of economic devastation if it attacked Kurdish forces in Syria after a planned pullout of U.S. forces. A month earlier, Trump had abruptly announced his intention to withdraw all American troops from Syria, sowing confusion among allies, though several hundred personnel have remained.

“United States Armed Forces will not support or be involved in the operation, and United States forces, having defeated the ISIS territorial ‘Caliphate,’ will no longer be in the immediate area,” the White House said in the statement released late Sunday after Trump spoke to Erdogan by phone.

The Trump administration said Turkey would take responsibility for any Islamic State fighters captured in the area over the past two years. It gave no details and it wasn’t immediately clear what, if any, plan the NATO allies had agreed to handle the detainees or how they would be transferred to Turkish custody.

Erdogan’s planned operation aims to claim areas from the Kurdish militant group YPG and create a buffer zone inside Syria where he aims to settle many of the 3.7 million Syrian refugees currently residing in Turkey.

The president is seeking more than $26 billion in aid to build houses for about 2 million Syrian refugees he hopes to resettle. That would alleviate the burden on Turkey’s economy -- and the criticism Erdogan has faced at home -- at a time when a possible Syrian government offensive on the last rebel bastion of Idlib fuels fears of another wave of refugees heading north.

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“The housing project envisaged by the Turkish authorities” is a “possible bonanza for Turkish construction firms close to the AKP,” Wolfango Piccoli, co-president at Teneo Intelligence, said Monday in a note, referring to Erdogan’s ruling party.

For Ankara, the militant YPG is a mortal enemy because of its links to another Kurdish separatist movement that Turkey has been fighting for over three decades. The SDF, of which YPG is a key component, said via Twitter that a Turkish incursion would “reverse the successful effort to defeat” the Islamic State. About 12,000 Islamic State militants and about 70,000 of their family members, who are currently in jails or camps, may be freed by “ISIS cells,” SDF said.

“Allowing Turkey to move into Northern Syria is one of the most destabilizing moves we can do in the Middle East. The Kurds will never trust America again. They will look for new alliances or Independence to protect themselves,” Ruben Gallego, Democratic Congressman for Arizona, said on Twitter.

Erdogan said U.S. forces have started to withdraw from the border area following his phone call with Trump. Speaking at the airport in Ankara before his departure for Serbia, he said the number of Islamic State suspects in jails, including foreigners from Germany and France, are “exaggerated” and that a study was underway to determine steps to “speedily” process them. He did not elaborate.

Wither Islamic State

Trump has characterized the U.S. military effort in Syria as a total victory, one that he has regularly touted on the campaign trail ahead of the 2020 election. At the same time, he’s insisted that the U.S. would bear no responsibility for any Islamic State detainees and gone so far as to threaten to release those fighters back to their countries of origin, which include several European nations whose governments have refused to take them.

“The United States Government has pressed France, Germany, and other European nations, from which many captured ISIS fighters came, to take them back, but they did not want them and refused,” the White House statement read. “The United States will not hold them for what could be many years and great cost to the United States taxpayer.”

Brett McGurk, a former U.S. envoy to the global coalition against Islamic State, said on Twitter that the White House statement “demonstrates a complete lack of understanding of anything happening on the ground.”