The Trump administration has ordered an immediate withdrawal of U.S. forces from Syria, and it remains unclear whether any troops will remain on the ground there.

The news seemingly caught the Pentagon and even local Syrian allies off-guard and runs counter to statements of many senior national security leaders.

After multiple media reports that a total withdrawal is underway, the White House said in a statement that the U.S.-led coalition is transitioning to the next phase of the campaign.

“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," White House Press Secretary Sarah Sanders said. "These victories over ISIS in Syria do not signal the end of the Global Coalition or its campaign. We have started returning United States troops home as we transition to the next phase of this campaign."

Sanders did not provide a timeline for a departure, nor elaborate on which troops were coming home.

Smoke billows after bombings in the Deir Ezzor province as a vehicle of the Syrian Democratic Forces (SDF) stops in Hajin, in the Deir Ezzor province, eastern Syria, on Dec. 15, 2018. (Delil Souleiman/AFP via Getty Images)
Smoke billows after bombings in the Deir Ezzor province as a vehicle of the Syrian Democratic Forces (SDF) stops in Hajin, in the Deir Ezzor province, eastern Syria, on Dec. 15, 2018. (Delil Souleiman/AFP via Getty Images)

The move comes as Turkish leaders are threatening an invasion of Syria that could pit U.S. advisers and U.S.-backed local forces against Turkey, a NATO ally.

Meanwhile, the Pentagon has remained much more opaque about the next step.

“We have started the process of returning U.S. troops home from Syria as we transition to the next phase of the campaign,” chief Pentagon spokeswoman Dana White said in a statement Wednesday. "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.”

President Donald Trump tweeted Wednesday morning that the U.S. has defeated the Islamic State in Syria, which was “my only reason for being there," he said.

Defense Secretary Jim Mattis and other top military officials tried to discourage Trump from ordering the withdrawal, saying it was a bad idea and would risk ceding control of Syria to Iran and Russia or give the Islamic State a chance to regroup, according to the New York Times.

The Wall Street Journal broke the news that the withdrawal is already being prepared. The Associated Press reported that planning for the pullout has already begun and troops will begin leaving as soon as possible.

The Pentagon initially declined to confirm the media reports, issuing an earlier statement Wednesday morning that made no mention of withdrawal.

“At this time we continue to work by, with and through our partners in the region," said Col. Rob Manning, a Pentagon spokesman early Wednesday morning.

Last week, Trump administration officials appeared to brush aside the idea of a withdrawal.

“The military mission is the enduring defeat of ISIS,” Brett McGurk, Trump’s special presidential envoy for the Global Coalition to Defeat ISIS, told reporters at a Dec. 11 press conference. “We have obviously learned a lot of lessons in the past, so we know that once the physical space is defeated, we can’t just pick up and leave.

“So we’re prepared to make sure that we do all we can to ensure this is enduring … Nobody is saying that (ISIS is) going to disappear. Nobody is that naive. So we want to stay on the ground and make sure that stability can be maintained in these areas.”

This picture shows a U.S. Army vehicle supporting the Syrian Democratic Forces (SDF) in Hajin, in the Deir Ezzor province, eastern Syria, on Dec. 15, 2018. (Delil Souleiman/AFP via Getty Images)
This picture shows a U.S. Army vehicle supporting the Syrian Democratic Forces (SDF) in Hajin, in the Deir Ezzor province, eastern Syria, on Dec. 15, 2018. (Delil Souleiman/AFP via Getty Images)

The Syriac Military Council, a small U.S.-backed militia in Syria, told Military Times on Wednesday that they had not heard of a planned withdrawal from their U.S. allies prior to the media reports.

Officials estimate there are about 2,000 U.S. troops in Syria, but the precise number is not disclosed publicly. Those U.S. forces are spread across the region in a network of forward operating bases and small units of combat advisers embedded with local allies, mainly Syrian Kurds.

Ten bases, including two with air strips, where U.S. troops have previously operated in northern Syria were identified in July 2017 on a map published by Turkey’s state-run news agency. The Pentagon expressed operational security concerns with Turkey over the identification of those outposts, and they may have shifted since then.

U.S.-backed local forces have eliminated ISIS' last major holdout in the Hajin pocket, near the Syria-Iraq border. However, ISIS still has several thousand fighters in the Middle Euphrates River Valley.

ISIS has also shown an ability to launch major counter-offensives whenever U.S. air power is not in the area, such as an October incident when U.S. aircraft were grounded due to a sandstorm.

U.S. troops in Syria have worked alongside a mix of Arab and Kurdish local militias that combine to form the Syrian Democratic Forces.

Turkey has long been angered by U.S. support for Kurdish fighters, which Turkey says are members of the Kurdistan Workers' Party, an ethnically Kurdish terrorist group that has waged an insurgency in southeast Turkey for decades.

Turkish President Recep Tayyip Erdogan said earlier this week that U.S. President Donald Trump had “responded positively” to Erodgan’s demands to remove Kurdish militias from Manbij, a region where U.S. troops are posted in northern Syria.

On Monday, though, U.S. Ambassador on Syrian Affairs Jim Jeffrey appeared to contradict Erdogan.

“We think that any offensive into northeast Syria by anyone is a bad idea, and that was a position that I conveyed when I was in Ankara, that everybody from the president on down has conveyed,” Jeffrey said, according to Voice of America.

Pentagon leaders have also repeatedly stated that the U.S. must maintain a presence to ensure a lasting defeat of ISIS, as well as prevent the movement of Iranian proxy forces.

Trump’s national security adviser, John Bolton, backed that school of thought this fall.

“We’re not going to leave as long as Iranian troops are outside Iranian borders, and that includes Iranian proxies and militias,” Bolton said in September.

Another dynamic in the whole situation is the tensions between U.S. and Russian forces in Syria. Americans and Russian mercenaries have reportedly exchanged gunfire on more than one occasion in the country.

“There have been various engagements, some involving exchange of fire, some not,” U.S. Ambassador James Jeffrey said in late November. “Again, we are continuing our mission there and we are continuing to exercise our right of self-defense.”

Deputy Editor Leo Shane III contributed to this report.