WASHINGTON, Dec. 19 (Xinhua) -- The Trump administration announced on Wednesday that it has started returning U.S. troops home from Syria after claiming a victory in the fight against the Islamic State (IS) without revealing any detailed timetable.

"We have started returning United States troops home as we transition to the next phase of this campaign," said White House Spokesperson Sarah Sanders in a statement, claiming that America has "defeated the territorial caliphate."

Pentagon later echoed the White House claim, saying that the U.S. military has already begun the process of bringing the U.S. forces back while emphasizing the continuity of the campaign.

"The Coalition has liberated ISIS (the IS)-held territory, but the campaign against ISIS is not over," the Pentagon spokeswoman Dana White said in a tweet.

All U.S. State Department personnel are being evacuated from Syria within 24 hours, reported Reuters after the White House announcement, citing an anonymous administration official.

The official also revealed that the time-frame for the troops withdrawal from the war-torn Arab country is expected to be between 60 and 100 days, according to the report.

However, a senior Trump administration official, during a background briefing held on Wednesday afternoon, did not directly answer reporters' questions on how the administration intends to withdraw the troops or whether there will a deadline for that.

Sanders' statement came about one hour after U.S. President Donald Trump hinted in a tweet about imminent U.S. troop withdrawal from Syria.

"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency," Trump tweeted on Wednesday morning.

Earlier on the day, U.S. media cited anonymous officials as saying that the United States is planning a "rapid" and "full" withdrawal of troops from Syria.

Currently, there are more than 2,000 U.S. soldiers deployed in Syria.

Trump has long voiced his desire to bring the U.S. troops back home when possible, while senior administration officials including Defense Secretary James Mattis have advocated for a longer-term military deployment in Syria to secure a victory against the IS.

Meanwhile, the Trump administration's claim about victory against the IS has been questioned.

"I strongly disagree. It has morphed into other forms of extremism and the threat is very much alive," tweeted British Defense Minister Tobias Ellwood, following Trump's claim on the social media.

Senior Republican Senator Lindsey Graham warned "devastating consequences" for U.S. troops quitting Syria.

"An American withdrawal at this time would be a big win for ISIS, Iran, Bashar al Assad of Syria, and Russia," the lawmaker from the state of South Carolina said in a statement on Wednesday.

The senator also noted that the withdrawal will "put the Kurds and all those who came to America's aid in destroying ISIS at tremendous risk."

The U.S. has been backing the Kurdish-led Syrian Democratic Forces (SDF) for a long time in the battles against the IS, managing to make important achievement in defeating IS in its de facto capital of Raqqa in northern Syria in 2017.

However, Turkey has for long been outright about its enmity to the Kurdish forces.