The U.S.-led coalition against Islamic State has begun the process of withdrawing from Syria, its spokesman said on Friday, affirming the start of a pullout that has been clouded by mixed messages from U.S. officials.

Members of the Syrian Democratic Forces (SDF), trained by the US-led coalition, participate in the graduation ceremony of their first regiment in al-Kasrah, in the suburb of eastern Syrian city of Deir Ezzor, on May 21, 2018.

President Donald Trump announced last month the decision to withdraw 2,000 U.S. troops who have deployed to Syria in support of the Kurdish-led Syrian Democratic Forces (SDF) militia in the fight against Islamic State.

The coalition "has begun the process of our deliberate withdrawal from Syria. Out of concern for operational security, we will not discuss specific timelines, locations or troop movements," Colonel Sean Ryan said.

Washington's Kurdish allies in northern Syria fear the withdrawal will open the way for Turkey to mount a long-threatened offensive against their area. Turkey views the dominant Syrian Kurdish groups as a national security threat.

U.S. National Security Adviser John Bolton suggested on Tuesday that protecting the Kurds would be a pre-condition of the U.S. withdrawal, drawing a rebuke from Turkish President Tayyip Erdogan who called his comments "a serious mistake".

But U.S. Secretary of State Mike Pompeo said on Thursday the withdrawal would not be scuppered despite the Turkish threats.