WASHINGTON: The United States said yesterday it has begun withdrawing US forces from Syria as US officials said the United States was considering pulling out all its troops as it winds up its campaign to retake territory once held by Islamic State. "We have started returning United States troops home as we transition to the next phase of this campaign," White House spokeswoman Sarah Sanders said in a statement issued after President Donald Trump tweeted that "We have defeated ISIS in Syria, my only reason for being there."

The stunning move will have extraordinary geopolitical ramifications and throws into question the fate of US-backed Kurdish fighters who have been tackling IS militants. Only last week Brett McGurk, the special envoy to defeat IS, said: "nobody is declaring a mission accomplished." It was not immediately clear from Sanders' statement whether all of the roughly 2,000 US troops in the country would leave and 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.

A decision to pull out completely, if confirmed, would upend assumptions about a longer-term US military presence in Syria, which senior US officials have advocated to help ensure Islamic State cannot reemerge. It could also undercut US 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 22 million population. The US State Department is evacuating all of its personnel from Syria within 24 hours, a US official told Reuters.

Reports of a full US 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 yesterday showed he saw no further grounds for remaining. US officials, who spoke to Reuters on condition of anonymity, did not disclose details about the deliberations on the troop withdrawal, and the timing was not immediately clear.

But one official told Reuters that partners and allies had been consulted. Two US officials said a decision to withdraw had already been reached but that could not be immediately confirmed. A US official said Trump's decision was finalized Tuesday. "Full withdrawal, all means all," the official said when asked if the troops would be pulled from all of Syria. The official would not provide a timeline for a withdrawal, saying only: "We will ensure force protection is adequately maintained, but as quickly as possible." The Pentagon would not confirm the US troop pullout. "At this time, we continue to work by, with and through our partners in the region," Pentagon spokesman Colonel Rob Manning said.

Republican US Senator Lindsey Graham, often a Trump ally, said a withdrawal would have "devastating consequences" for the United States in the region and throughout the world. "An American withdrawal at this time would be a big win for ISIS, Iran, (President) Bashar Al-Assad of Syria, and Russia," Graham said in a statement, using the acronym ISIS for Islamic State.

Charles Lister, a senior fellow at the Middle East Institute, called the decision "extraordinarily short-sighted and naive". "This move will look like a 'withdrawal,' not a 'victory,' and yet more evidence of the dangerous unpredictability of the US president," Lister said. "This is not just a dream scenario for ISIS, but also for Russia, Iran and the Assad regime, all of whom stand to benefit substantially from a US withdrawal."

Many of the remaining US 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 Islamic State in Syria, but has also outraged NATO ally Turkey, which views Kurdish YPG forces in the alliance as an extension of a militant group fighting inside Turkey.

The deliberations on US troops come as Ankara threatens a new offensive in Syria. To date, US forces in Syria have been seen as a stabilizing factor in the country and have somewhat restrained Turkey's actions against the SDF. A complete withdrawal of US troops from Syria would still leave a sizeable US military presence in the region, including about 5,200 troops across the border in Iraq. Much of the US campaign in Syria has been waged by warplanes flying out of Qatar and other locations in the Middle East.

Still, Defense Secretary Jim Mattis and US State Department officials have long fretted about leaving Syria before a peace agreement can be reached to end the brutal civil war. Islamic State is also widely expected to revert to guerilla tactics once it no longer holds territory. A US withdrawal could open Trump up to criticism if Islamic State reemerged. Trump has previously lambasted his predecessor, Barack Obama, for the withdrawal of US forces from Iraq that preceded an unraveling of the Iraqi armed forces. Iraqi forces collapsed in the face of Islamic State's advance into the country in 2014.

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.

Islamic State declared its so-called "caliphate" in 2014 after seizing large swathes of Syria and Iraq. The hardline Islamist group established its de facto capital in the Syrian city of Raqqa, using it as a base to plot attacks in Europe. According to US estimates, the group oversaw about 100,000 sq km of territory, with about 8 million people under Islamic State control. It had estimated revenues of nearly one billion dollars a year.

McGurk, the US special envoy for the global coalition to defeat Islamic State, said last week that the group was down to its last 1 percent of the territory it once held in its self-styled "caliphate." The group has no remaining territory in Iraq. Hajin, the group's last major stronghold in Syria, is close to being seized by US-backed SDF forces. After losing Hajin, Islamic State will control a diminishing strip of territory along the eastern bank of the Euphrates River in the area where US-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 US officials have warned that taking back the group's territory would not be the same as defeating it. "Even as the end of the physical caliphate is clearly now coming into sight, the end of ISIS will be a much more long-term initiative," McGurk told a State Department briefing on Dec 11. US Marine General Joseph Dunford, the chairman of the Joint Chiefs of Staff, cautioned earlier in December that the United States had trained only about 20 percent of Syrian forces required to stabilize areas captured from Islamic State. - Agencies