Updated at 1:05 p.m. ET

The White House announced late Sunday that Turkey is ready to launch an offensive in northern Syria and that U.S. forces will stand aside, renewing fears that America is abandoning Kurdish allies who stood on the front line in the years-long fight against ISIS.

A two-paragraph statement released by White House press secretary Stephanie Grisham said that President Trump and Turkey's President Recep Tayyip Erdogan had spoken by telephone and that "Turkey will soon be moving forward with its long-planned operation in northern Syria."

"The 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," it added.

The abrupt announcement immediately raised concerns among U.S. allies, who worry that Turkey could now be given a free hand to move against Kurdish fighters whom Washington views as allies but Ankara considers to be terrorists who are allied with Kurdish separatists in Turkey.

The Kurdish-led Syrian Democratic Forces called Trump's decision"shocking," adding that it "has made people in the Northeast lose hope."

In an apparent attempt to ease those concerns, and possibly clarify the White House's announcement, the Pentagon said on Monday that it has not endorsed a Turkish military offensive over the border.

"The Department of Defense made clear to Turkey — as did the President — that we do not endorse a Turkish operation in Northern Syria. The U.S. Armed Forces will not support, or be involved in any such operation," said Jonathan Hoffman, assistant to the Secretary of Defense for Public Affairs.

In a tweet, Trump said, "if Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey (I've done before!)."

U.S. forces on the ground in Syria recruited and trained the Syrian Democratic Forces, or SDF, which bore the brunt of the fighting on the ground against Islamic State fighters, assisted by U.S. airstrikes.

Early last year, Turkey launched an offensive in Kurdish-dominated northwestern Syria, slowly pushing Kurdish militia and civilians into the city of Afrin, which Turkey captured in March 2018.

In December, Trump abruptly announced that U.S. troops would be withdrawn from Syria — a move that elicited praise from Turkey but sparked criticism among Washington's allies and within the president's own Cabinet, where the move prompted the resignation of then-Defense Secretary Jim Mattis. Within days, then-National Security Adviser John Bolton clarified that the U.S. drawdown would be conditioned on the defeat of the Islamic State and the safety of the Kurds.

The U.S. has a total of about 1,000 troops in northern Syria and it wasn't immediately clear whether the White House statement meant they would be totally withdrawn from the country or only from the immediate area of Turkish operations.

Erdogan has threatened for months to launch an assault against Kurds in northern Syria. But U.S. lawmakers on both sides of the aisle have warned that allowing Ankara a free hand against the Kurds would send a troubling message about U.S. commitment to its allies.

Judging by what has emerged from the conversation between Trump and Erdogan, it seems that each leader is now hoping to resolve their own nagging issues related to Syria.

As NPR's Daniel Estrin reports, Erdogan is facing domestic pressure over his handling of more than 3 million Syrian refugees who are in Turkey. With the incursion, Estrrin says, Turkey hopes to "create a so-called safe zone to resettle the refugees" outside its own borders.

And for Trump, there's the conundrum of what to do with foreign fighters who were taken captive in the battle against ISIS militants.

Throughout the conflict in Syria, the Islamic State used social media to recruit fighters from abroad, scoring particular success in parts of Europe. According to Ambassador James Jeffrey, the State Department envoy to the coalition fighting the Islamic State, Kurdish forces in northern Syria are holding about 2,500 foreign fighters.

The White House statement said the U.S. "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 United States will not hold them for what could be many years at great cost to the United States taxpayer."

Kurdish forces, not the U.S., have kept ISIS fighters, sympathizers and family members in makeshift camps in northern Syria.

The White House statement said, "Turkey will now be responsible for all ISIS fighters in the area captured over the past two years in the wake of the defeat of the territorial 'Caliphate' by the United States."

Copyright 2021 NPR. To see more, visit https://www.npr.org.