A fighter from the Syrian Democratic Forces, SDF, stands inside a post where U.S. troops were based, in Tel Abyad town, at the Syrian-Turkish border, Syria, Monday, Oct. 7, 2019. The SDF, a U.S.-backed Kurdish-led force accused Washington Monday of failing to abide by its commitments by withdrawing from northeast Syria ahead of a Turkish invasion that the Kurds say will overturn five years of achievements in the battle against the Islamic State group. (AP Photo/Ahmad Baderkhan)

BEIRUT (AP) — Syria's Kurds accused the U.S. of turning its back on its allies and risking gains made in the fight against the Islamic State group as American troops began pulling back on Monday from positions in northeastern Syria ahead of an expected Turkish assault.

U.S. President Donald Trump's abrupt decision to stand aside — announced by the White House late Sunday — infuriated Kurds, who stand to lose the autonomy they gained in the course of Syria's civil war.

The Kurdish force pledged to fight back, raising the potential for an eruption of new warfare in Syria. "We will not hesitate for a moment in defending our people" against Turkish troops, the Kurdish-led Syrian Democratic Forces said in a statement, adding that it has lost 11,000 fighters in the war against IS in Syria.

As many as 300,000 people could immediately be driven from their homes in northeast Syria if Turkey launches its offensive, the International Rescue Committee warned Monday.

Turkish President Recep Tayyip Erdogan has threatened for months to launch the military operation across the border. He views the Syria Kurdish forces as terrorists and a threat to his country as Ankara has struggled with a Kurdish insurgency within Turkey.

Ankara has been demanding a "safe zone" stretching the length of northern Syria along Turkey's southern border to be patrolled by Turkish troops and their allied Syrian forces. That would put a significant portion of Syria's Kurdish population under effective Turkish control.

Erdogan on Monday said American troops have started pulling back following his conversation with Trump the night before. He did not elaborate on the planned Turkish incursion but said Turkey was determined to halt what it perceives as threats from the Syrian Kurdish fighters.

The SDF issued a sharp condemnation of the American move. "The American forces did not abide by their commitments and withdrew their forces along the border with Turkey," it said.

A U.S. official confirmed that American troops were already moving out of the security zone area, which includes the Syrian towns of Ras al-Ayn and Tal Abyad. That official was not authorized to speak for the record and was granted anonymity to comment.

A video posted by a Kurdish news agency showed a convoy of American armored vehicles apparently heading away from the border area of Tal Abyad.

America's rivals, including Iran, Russia and the Syrian government, stand to gain from a U.S. troop withdrawal from the oil-rich region in the north. Iran's Foreign Minister Javad Zarif tweeted: "US is an irrelevant occupier in Syria — futile to seek its permission or rely on it for security."

In Moscow, Kremlin spokesman Dmitry Peskov said Moscow realizes Turkey's need to ensure its security, but noted that "it's necessary to respect Syria's territorial and political integrity." Peskov wouldn't comment on whether the U.S. withdrawal could push the Kurds to seek a dialogue with Damascus.

Russia and Iran have helped Syrian President Bashar Assad reclaim control over most of the country following a devastating eight-year civil war.

Abdulkarim Omar, a senior official in the Kurdish self-rule administration, said they had been expecting the U.S. decision to withdraw and have made preparations for it. He didn't elaborate. But he warned that securing facilities holding IS militants would be jeopardized if an offensive begins because forces would be deployed there.

"We have been flexible even in dealing with Russia, which may play a role in the political resolution. We were flexible even in regards to Damascus," he said. "But what happened today is illogical."

The Kurdish-led SDF has been the main U.S.-backed force in Syria in the fight against IS. In March, the SDF captured the last sliver of land held by the extremists, marking the end of the so-called caliphate that was declared by IS's leader Abu Bakr al-Baghdadi in 2014.

The U.S. and Turkey had been working on a compromise "security mechanism" for the border region that the Kurds had hoped would avert any Turkish offensive. Since August, joint U.S and Turkish aerial and ground patrols had started in a 125-kilometer (78-mile) zone. The SDF had cooperated, removing fortifications from the areas and withdrawing with heavy weapons.

But vital details of the mechanism were still being worked out, and Ankara had repeatedly expressed its impatience, threatening an attack.

Mustafa Bali, the SDF spokesman, tweeted that his group had not been not expecting the U.S. to protect northeastern Syria. "But people here are owed an explanation regarding the security mechanism deal and destruction of fortifications," he said.

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Associated Press writers Lolita C. Baldor and Zeke Miller in Washington, and Sarah El Deeb and Zeina Karam in Beirut contributed to this report.

In this image provided by Hawar News Agency, ANHA, U.S. military vehicles travel down a main road in northeast Syria, Monday, Oct. 7, 2019. U.S.-backed Kurdish-led forces in Syria said American troops began withdrawing Monday from their positions along Turkey's border in northeastern Syria, ahead of an anticipated Turkish invasion that the Kurds say will overturn five years of achievements in the battle against the Islamic State group. (ANHA via AP)
FILE - In this Wednesday, July 11, 2018, file photo, President Donald Trump, left, talks with Turkey's President Recep Tayyip Erdogan, as they arrive together for a family photo at a summit of heads of state and government at NATO headquarters in Brussels. The White House says Turkey will soon invade Northern Syria, casting uncertainty on the fate of the Kurdish fighters allied with the U.S. against in a campaign against the Islamic State group. (AP Photo/Pablo Martinez Monsivais, File)
In this image provided by Hawar News Agency, ANHA,  a line of U.S. military vehicles travel down a main road in northeast Syria, Monday, Oct. 7, 2019. U.S.-backed Kurdish-led forces in Syria said American troops began withdrawing Monday from their positions along Turkey's border in northeastern Syria, ahead of an anticipated Turkish invasion that the Kurds say will overturn five years of achievements in the battle against the Islamic State group. (ANHA via AP)
In this Sept. 21, 2019, photo, released by the U.S. Army, a U.S. soldier oversees members of the Syrian Democratic Forces as they demolish a Kurdish fighters' fortification as part of the so-called
A Turkish n armored vehicles patrol as they conduct a joint ground patrol with American forces in the so-called
Turkish and American armored vehicles patrol as they conduct joint ground patrol in the so-called
A Turkish n armored vehicles patrol as they conduct a joint ground patrol with American forces in the so-called
In this Sunday, Oct. 6, 2019 photo, Turkish forces artillery pieces are driven to their new positions near the border with Syria in Sanliurfa province, Turkey. U.S.-backed Kurdish-led forces in Syria said American troops began withdrawing Monday from their positions along Turkey's border in northeastern Syria, ahead of an anticipated Turkish invasion that the Kurds say will overturn five years of achievements in the battle against the Islamic State group. (DHA via AP)
In this Sunday, Oct. 6, 2019 photo, Turkish forces artillery pieces are seen on their new positions near the border with Syria in Sanliurfa province, Turkey. U.S.-backed Kurdish-led forces in Syria said American troops began withdrawing Monday from their positions along Turkey's border in northeastern Syria, ahead of an anticipated Turkish invasion that the Kurds say will overturn five years of achievements in the battle against the Islamic State group. (DHA via AP)
Kurdish women flash victory signs and shout slogans as they protest against possible Turkish military operation on their areas, at the Syrian-Turkish border, in Ras al-Ayn, Syria, Monday, Oct. 7, 2019. Syria's Kurds accused the U.S. of turning its back on its allies and risking gains made in the fight against the Islamic State group as American troops began pulling back on Monday from positions in northeastern Syria ahead of an expected Turkish assault. (AP Photo)
Kurdish women hold a banner with Arabic that read,
A fighter from the Syrian Democratic Forces, SDF, flashes the victory sign during a demonstration against possible Turkish military operation on their areas, at the Syrian-Turkish border, in Ras al-Ayn, Syria, Monday, Oct. 7, 2019. Syria's Kurds accused the U.S. of turning its back on its allies and risking gains made in the fight against the Islamic State group as American troops began pulling back on Monday from positions in northeastern Syria ahead of an expected Turkish assault. (AP Photo)
Most of Syria has returned to government control after eight years of war.;
Kurdish citizens wave their group's flags, as they protest against possible Turkish military operation on their areas, at the Syrian-Turkish border, in Ras al-Ayn, Syria, Monday, Oct. 7, 2019. Syria's Kurds accused the U.S. of turning its back on its allies and risking gains made in the fight against the Islamic State group as American troops began pulling back on Monday from positions in northeastern Syria ahead of an expected Turkish assault. (AP Photo)
A fighter from the Syrian Democratic Forces, SDF, stands guard inside a post where U.S. troops were based, in Tal Abyad town, at the Syrian-Turkish border, Syria, Monday, Oct. 7, 2019. The SDF, a U.S.-backed Kurdish-led force accused Washington Monday of failing to abide by its commitments by withdrawing from northeast Syria ahead of a Turkish invasion that the Kurds say will overturn five years of achievements in the battle against the Islamic State group. (AP Photo)
A fighter from the Syrian Democratic Forces, SDF, stands guard inside a post where U.S. troops were based, in Tal Abyad town, at the Syrian-Turkish border, Syria, Monday, Oct. 7, 2019. The SDF, a U.S.-backed Kurdish-led force accused Washington Monday of failing to abide by its commitments by withdrawing from northeast Syria ahead of a Turkish invasion that the Kurds say will overturn five years of achievements in the battle against the Islamic State group. (AP Photo)