BEIRUT - The U.S. military said Friday it has begun withdrawing troops from Syria, initiating a drawdown that has blindsided allies and sparked a scramble for control of the areas that American troops will leave.

U.S. forces have "begun the process of our deliberate withdrawal from Syria," Col. Sean Ryan, the spokesman for the U.S.-led coalition against the Islamic State said. "Out of concern for operational security, we will not discuss specific timelines, locations or troop movements."

President Donald Trump's Dec. 19 announcement that he was moving to disentangle U.S. troops from Syria's complex battlefield sparked fears that the move might undo efforts to defeat the Islamic State's final remnant in Syria.

It also marked the culmination of years of criticism by Trump over Washington's role in foreign wars. In public statements, he had repeatedly suggested that he wanted to bring American troops back home.

But following a backlash from Republicans and Democrats in Congress and the resignation of Defense Secretary Jim Mattis, Trump said the troops would be withdrawn "slowly."

Extending the timeline further, national security adviser John Bolton said on Sunday that the pullout was conditional on the defeat of the last remnants of the Islamic State and guarantees from Turkey that it would not attack Kurdish forces aligned with the United States.

But Erdogan made clear this week that he would not agree to those terms and lashed out at Trump's aide, saying "It is not possible for us to swallow the message Bolton gave."

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said sources in Syria's northeastern province of Hasakah reported the departure of about 10 armored vehicles from a military base in the town of Rmelan late Thursday.

That report could not be independently confirmed Friday morning, and it was unclear whether the movement indicated a drawdown or was just a planned rotation. There are some 2,000 U.S. troops stationed across northeastern Syria.

Foreign allies, including the Kurdish-led force that has spearheaded Washington's fight against the Islamic State, say they had not received warning about the withdrawal announcement, and administration officials initially offered differing timetables for its completion.

Representatives of the Syrian Democratic Forces, a military coalition dominated by Kurdish fighters, declined to comment Friday, suggesting instead that the U.S. military should explain its plans.

Hudson reported from Abu Dhabi, United Arab Emirates.