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The United States' main ally in Syria on Thursday rejected President Donald Trump's claim that Islamic State militants have been defeated and warned that the withdrawal of American troops would lead to a resurgence of the extremist group.

The Kurdish-led Syrian Democratic Forces said in a statement that a premature U.S. troop pullout would have dangerous repercussions and a destabilizing effect on the region.

"The war against Islamic state has not ended and the Islamic State has not been defeated," the statement said. It was the first official comment by the group on Trump's surprise announcement Thursday.

Trump's decision to withdraw troops from Syria has rattled Washington's Kurdish allies, who are its most reliable partner in Syria and among the most effective ground forces battling IS. With U.S. air support, the Kurds drove IS from much of northern and eastern Syria in a costly four-year campaign.

Kurdish officials and commanders met into the night, discussing their responses to the decision, local residents said Thursday.

Arin Sheikmos, a Kurdish journalist and commentator, said "we have every right to be afraid."

"If the Americans pull out and leave us to the Turks or the (Syrian) regime our destiny will be like the Kurds of Iraqi Kurdistan in 1991 — million of refugees, there will be massacres. Neither the regime, not Iran nor Turkey, will accept our presence here," he told the AP.

Trump's decision to withdraw is widely seen as an abandonment of a loyal ally. Turkish President Recep Tayyip Erdogan has repeatedly threatened to launch a new offensive against the Kurds. The threat from Turkey could drive the Kurds into the arms of Syrian President Bashar Assad, and by extension Iran and Russia.

Turkey views the People's Protection units, or YPG, the main component of the Syrian Democratic Forces, as a terrorist group and an extension of the insurgency within its borders. U.S. support for the group has strained ties between the two NATO allies.

"This is expected," Ebrahim Ebrahim, a Syrian Kurd based in Europe, said of the pullout. "But it is not just treason to the Kurds or the people of Syria but to democracy, to morals, if this is true. Yes, true, we fought for ourselves, but we also fought for democracies all over the world," he added.

A Syrian member of parliament, Peter Marjana, said Thursday that a U.S. pullout would be a "recognition that Syria has won."

Trump's contention that IS has been defeated contradicted his own experts' assessments and shocked his party's lawmakers, who called his decision rash and dangerous.

The U.S. began airstrikes against IS in Syria in 2014, and later sent in ground troops to aid Kurdish forces. Trump abruptly declared their mission accomplished in a tweet.