The United States said on Wednesday it has begun withdrawing its forces from Syria, while American officials said the U.S. was considering pulling out all its troops as it winds up its campaign to retake territory once held by Islamic State in Iraq and Syria.

"We have started returning United States troops home as we transition to the next phase of this campaign," White House spokesperson Sarah Huckabee Sanders said in a statement after U.S. President Donald Trump claimed in a tweet that American forces had vanquished ISIS in the country. He later added a video of himself declaring that the U.S. has beaten ISIS "badly."

It wasn't immediately clear from Sanders's statement whether all of the roughly 2,000 U.S. troops in the country would leave or if so, by when.

Sanders suggested that the United States would remain engaged to some degree.

"The United States and our allies stand ready to re-engage at all levels to defend American interests whenever necessary, and we will continue to work together to deny radical Islamist terrorists territory, funding, support," she said.

After historic victories against ISIS, it’s time to bring our great young people home! <a href="https://t.co/xoNjFzQFTp">pic.twitter.com/xoNjFzQFTp</a> —@realDonaldTrump

A decision to withdraw completely, if confirmed, would upend assumptions about a longer-term U.S. military presence in Syria, which U.S. Defence Secretary Jim Mattis and other senior U.S. officials have advocated to help ensure ISIS cannot re-emerge.

It could also undercut U.S. leverage in the region and undermine diplomatic efforts to end a civil war in Syria that has killed hundreds of thousands of people and displaced around half of the country's pre-war population of 22 million.

Timing 'hard to understand'

A pullout would allow other countries, like Iran, to increase their influence in Syria, experts said.

"If we withdraw, then who fills the vacuum, who is able to stabilize — and that is the million-dollar question," said Andrew Tabler, a Syria specialist at the Washington Institute for Near East Policy think-tank.

"The timing is hard to understand," Tabler said.

Israeli Prime Minister Benjamin Netanyahu, who remains concerned about Iranian efforts in the area, reacted in a noncommittal fashion after talking with Trump by telephone.

"This is, of course, an American decision," he said. No matter what, he said, "we will safeguard the security of Israel and protect ourselves from this arena."

The U.S. State Department is evacuating all of its personnel from Syria within 24 hours, a U.S. official told Reuters.

Criticism from all sides

Reports of a full U.S. military withdrawal drew immediate criticism, including from some of Trump's fellow Republicans.

Trump has previously expressed a strong desire to bring troops home from Syria when possible, and his tweet on Wednesday showed he saw no further grounds for remaining.

Britain said Wednesday that the Islamic State remained a threat even though it held no territory, commenting after Trump said the militant group had been defeated in Syria.

"Much remains to be done and we must not lose sight of the threat they pose. Even without territory, [ISIS] will remain a threat," the Foreign Office said in a statement after the United States began withdrawing troops from Syria.

"As the United States has made clear, these developments in Syria do not signal the end of the Global Coalition or its campaign. We will continue to work with members of the Coalition on achieving this."

The timeline for the American withdrawal was not immediately clear and U.S. officials who spoke to Reuters on condition of anonymity did not disclose details about the deliberations, including who was involved. One official told Reuters that partners and allies had been consulted.

Pentagon spokesperson Dana White said in a statement that "the campaign against ISIS is not over" but that the department had "started the process of returning U.S. troops home" from Syria.

"For force protection and operational security reasons, we will not provide further details," the statement said.

Just last week, the U.S. special envoy to the anti-ISIS coalition, Brett McGurk, said U.S. troops would remain in Syria even after ISIS was driven from its strongholds.

Republican Sen. Lindsay Graham, who just returned from Afghanistan, said he was meeting with Defence Secretary Jim Mattis late in the day.

Graham, typically a Trump backer, said he was "blindsided" by the report and called the decision "a disaster in the making."

"The biggest winners in this are ISIS and Iran," he said.

Sen. Marco Rubio of Florida said a full and rapid removal of troops would be a "grave error with broader implications" beyond the fight against ISIS.

Many of the remaining U.S. troops in Syria are special operations forces working closely with an alliance of Kurdish and Arab militias known as the Syrian Democratic Forces, or SDF.

The partnership with the SDF over the past several years has led to the defeat of ISIS but outraged NATO ally Turkey, which views Kurdish YPG forces in the alliance as an extension of a militant group fighting inside Turkey.

New Turkish offensive?

The deliberations on U.S. troops come as Ankara threatens a new offensive in Syria. To date, U.S. forces in Syria have been seen as a stabilizing factor in the country and have somewhat restrained Turkey's actions against the SDF.

Just hours before the withdrawal decision became public, the State Department announced late Tuesday that it had approved the sale of a $3.5-billion US Patriot missile defence system to Turkey.

The Turks had complained that the U.S. was slow walking requests for air defences. They had signed a deal with Russia to buy a sophisticated system. Washington and Ankara's other NATO partners were strongly opposed to the Russia deal.

A complete withdrawal of U.S. troops from Syria would still leave a sizeable U.S. military presence in the region, including about 5,200 troops across the border in Iraq. Much of the U.S. campaign in Syria has been waged by warplanes flying out of Qatar and other locations in the Middle East.

Fighters with the People's Protection Units, or YPG, stand next to an outpost near the city of Kobani in northern Syria on Tuesday. Turkey has vowed to launch a new offensive against the YPG, which is the main component of a U.S.-allied force that drove ISIS militants out of much of eastern Syria. (Ugur Can/DHA via Associated Press)

Still, Mattis and U.S. State Department officials have long fretted about leaving Syria before a peace agreement can be reached to end that country's brutal civil war, which has killed hundreds of thousands of people and displaced around half of Syria's pre-war population of about 22 million.

In April, Mattis said: "We do not want to simply pull out before the diplomats have won the peace. You win the fight — and then you win the peace."

ISIS is also widely expected to revert to guerilla tactics once it no longer holds territory.

A U.S. withdrawal could open Trump up to criticism if ISIS re-emerges.

Trump has previously lambasted his predecessor, Barack Obama, for the withdrawal of U.S. forces from Iraq that preceded an unraveling of the Iraqi armed forces. Iraqi forces collapsed in the face of ISIS's advance into the country in 2014.

Hajin, ISIS's last major stronghold in Syria, is close to being seized by U.S.-backed SDF forces.

Fighters with the Syrian Democratic Forces inspect weapons and munitions recovered from Islamic State militants in Raqqa on Oct. 7, 2017. (Erik De Castro/Reuters)

After losing Hajin, the group will control a diminishing strip of territory along the eastern bank of the Euphrates River in the area where U.S.-backed operations are focused. The militants also control some desert terrain west of the river in territory otherwise controlled by the Damascus government and its allies.

But U.S. officials have warned that taking back the group's territory would not be the same as defeating it.

U.S. marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, also cautioned earlier in December that the United States had trained only about 20 per cent of Syrian forces required to stabilize areas captured from ISIS.