WASHINGTON, Dec. 23 (Xinhua) -- The departing U.S. Secretary of Defense James Mattis has signed the execute order detailing the U.S. troop withdrawal from Syria, reported U.S. media on Sunday.

The order clarified how and when the U.S. soldiers' exit from the war-torn country will occur, reported CNN citing anonymous defense officials.

The withdrawal is expected to start "in the next several weeks" and could last "several weeks," according to the report.

After claiming a victory in the fight against the Islamic State (IS), the Trump administration announced on Wednesday that it started pulling U.S. troops back from Syria without revealing any detailed timetable.

The decision was widely regarded as the last straw for the resignation of Mattis on Thursday.

Trump tweeted on Sunday that he has picked Deputy Secretary of Defense Patrick Shanahan as the acting Pentagon chief, forcing Mattis to step down earlier than planned.

Trump also tweeted that he had a phone conversation with the Turkish President Recep Tayyip Erdogan over Syria issues.

"We discussed ISIS, our mutual involvement in Syria, and the slow and highly coordinated pullout of the U.S. troops from the area," he said.

The United States has deployed more than 2,000 troops in Syria to fight the IS since 2015.