A convoy of US forces armoured vehicles drives near the village of Yalanli, on the western outskirts of the northern Syrian city of Manbij, on March 5, 2017.

Shortly after Trump's Twitter declaration, White House press secretary Sarah Huckabee Sanders confirmed in a statement that "We have started returning United States troops home as we transition to the next phase of this campaign."

Numerous Republican lawmakers reacted harshly to reports of the withdrawal, decrying the decision as a grave mistake that could even restore ISIS's strength.

That morning, news reports said the U.S. military was planning a complete and sudden withdrawal from Syria in a reversal of U.S. policy, as the declining threat of ISIS has been supplanted by increasing tensions with other world powers in the region.

On Wednesday evening , Trump sent a follow-up tweet, saying "it's time to bring our great young people home!"

Defeating the militant Islamic group in the region, he said, was "my only reason for being there during the Trump Presidency."

President Donald Trump declared in a tweet Wednesday that the U.S. mission to defeat the so-called Islamic State in Syria has been accomplished.

"Five years ago, ISIS was a very powerful and dangerous force in the Middle East, and now the United States has defeated the territorial caliphate," Sanders said. "These victories over ISIS in Syria do not signal the end of the Global Coalition or its campaign."

Sanders added that the U.S. and its allies "will continue to work together to deny radical Islamist terrorists territory, funding, support, and any means of infiltrating our borders."

Chief Defense Department Spokeswoman Dana White said in a statement that "the campaign against ISIS is not over," despite the transition out of Syria:

"The Coalition has liberated the ISIS-held territory, but the campaign against ISIS is not over. We have started the process of returning U.S. troops home from Syria as we transition to the next phase of the campaign. For force protection and operational security reasons we will not provide further details. We will continue working with our partners and allies to defeat ISIS wherever it operates."

A U.S. official told Reuters that all State Department personnel in Syria will be evacuated from the country within 24 hours, and that all U.S. forces will depart once that final operation against ISIS has been completed. The troops are expected to be withdrawn within 60 to 100 days, the official told Reuters.

The decision would take 2,000-plus U.S. servicemembers out of the country, ending the ground strategy against the Islamic State, a U.S. defense official told The Washington Post.

The move appears to contradict the existing stance on the conflict in Syria advocated by senior Trump administration officials. They called for a longer-term presence in the country and even to expand the mission beyond defeating ISIS. National security advisor John Bolton reportedly said in September that the U.S. was "not going to leave as long as Iranian troops are outside Iranian borders."

Before the White House's announcement Wednesday, Defense Department spokesman Col. Rob Manning told reporters, "At this time, we continue to work by, with and through our partners in the region."

The surprise pivot comes just days after Turkey reaffirmed to CNBC that it would not soften its rhetoric against the U.S.-backed Kurds in northern Syria. Turkish Foreign Minister Mevlut Cavusoglu said his country's military was "not risking American soldiers' lives" through its targeting of the Kurdish factions.

During a news briefing Wednesday morning, a spokeswoman for Russia's foreign ministry knocked the existing U.S. forces in Syria as "a dangerous obstacle to the path to" a peace settlement, Reuters reported. The spokeswoman also accused the U.S. of keeping its troops in Syria illegally.

The State Department and the Russian Foreign Ministry did not respond to CNBC's requests for comment on the reports.

Later Wednesday, Sen. Lindsey Graham, R-S.C., a close ally of the president, warned in four-tweet thread that removing the U.S. presence in Syria would be a mistake of "Obama-like" proportions.

Graham tweet

Graham added: "A decision to withdraw will also be viewed as a boost to ISIS desire to come back."

Graham tweet 2

In a Facebook livestream, Sen. Marco Rubio, R-Fla., said that "while ISIS has been significantly degraded ... it is not fair or wise to say that they have been defeated."

"I believe the decision was a grave error that will have incredible consequences, potentially not fully thought through," Rubio said.

"This is a bad idea because it actually, it goes against the fight against ISIS, and potentially helps ISIS," Rubio said, adding that "this decision runs the risk of triggering a broader conflict that could pull the United States in, but most certainly will impact our close ally in the region, Israel."

GOP Rep. Adam Kinzinger of Illinois, who currently serves as an Air National Guard pilot, responded to Trump more tersely: "This is simply not true."

Kinzinger tweet