WASHINGTON (REUTERS, AFP) - President Donald Trump has begun what will be a total withdrawal of United States troops from Syria, declaring on Wednesday (Dec 19) that they have succeeded in their mission to defeat the Islamic State in Iraq and Syria (ISIS) and were no longer needed in the country.

Mr Trump said on Wednesday that it was time for US troops to come home from Syria, after years of battling the ISIS militant group in the war-wracked country.

"We won," Mr Trump said in a video message posted on Twitter, declaring the ISIS militants defeated. "We've beaten them and we've beaten them badly. We've taken back the land... So our boys, our young women, our men - they're all coming back and they're coming back now."

His message came at the end of a day of relative surprise in Washington at the news that the 2,000 US troops on the ground in Syria would be withdrawn.

A US official said Mr Trump's decision was finalised on Tuesday.

In his video message, the Republican President - facing criticism on both sides of the political aisle - insisted the time was right. "They're getting ready. You're going to see them soon. These are great American heroes," Mr Trump said.

But the move could leave the US with few options to prevent a resurgence of ISIS. It could also undercut US leverage in the region and undermine diplomatic efforts to end the Syrian civil war, which is now in its eighth year.

News of a full withdrawal drew immediate criticism from some of Mr Trump's fellow Republicans, who said that leaving strengthened the hand of Russia and Iran, which both support Syrian President Bashar al Assad.

It may also leave exposed an alliance of Kurdish and Arab militias known as the Syrian Democratic Forces, or SDF, which has been among the most effective against ISIS but is under threat, as Turkey weighs a new offensive in Syria.

US commanders on the ground, who have developed strong ties to SDF leaders, had voiced concerns about what a fast withdrawal would mean for the US-backed forces and were surprised by the decision, US officials told Reuters, speaking on condition of anonymity.

"We have started returning United States troops home as we transition to the next phase of this campaign," White House spokesman Sarah Sanders said in a statement issued after Mr Trump tweeted: "We have defeated ISIS in Syria, my only reason for being there."

Mr Trump's predecessor, Mr Barack Obama, was slow to get involved in Syria's civil war, fearing being dragged into another open-ended foreign conflict like the one in Afghanistan. Hundreds of thousands of people have been killed in Syria and around half its pre-war 22 million population displaced.

But in a campaign to defeat ISIS in Syria, Mr Obama ordered air strikes from September 2014 and then troops into the country the following year.

One US official said Washington aimed to withdraw troops within 60 to 100 days and said the US State Department was evacuating all its personnel in Syria within 24 hours. A second official said they could leave even sooner.

TRUMP'S WARINESS

Mr Trump is wary of open-ended foreign conflicts and his decision on Syria raises questions about whether he may also reconsider the US war effort in Afghanistan, where American forces have been fighting since 2001.

Mr Trump reluctantly agreed to a troop increase last year but US officials have privately acknowledged a sense of urgency and are increasingly focused on securing a peace deal with a resurgent Taleban.

Some of Mr Trump's Republican allies in Congress railed against the pullout decision. US Senator Lindsey Graham, often a Trump ally but generally a foreign policy hawk, said a withdrawal would have "devastating consequences" for the US in the region and throughout the world.

"An American withdrawal at this time would be a big win for ISIS, Iran, Bashar al Assad of Syria, and Russia," Mr Graham said in a statement.

The surprise decision also raised eyebrows abroad.

A British defence minister said he strongly disagreed with Mr Trump that ISIS had been defeated in Syria. Israeli Prime Minister Benjamin Netanyahu said Israel would study the decision and would ensure its own security.

In Russia, TASS news agency quoted the Foreign Ministry as saying withdrawing US troops from Syria created prospects for a political settlement.

SPECIAL FORCES

Many of the remaining US troops in Syria are special operations forces working closely with the SDF.

The partnership with the SDF has helped in the defeat of ISIS in Syria but has outraged Nato ally Turkey, which views Kurdish YPG forces in the alliance as an extension of a separatist militant group fighting inside Turkey.

Ankara is threatening a new offensive in Syria. To date, US forces in Syria have been seen as a stabilising factor and have somewhat restrained Turkey's actions against the SDF.

A complete withdrawal of US troops from Syria would 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, Defence Secretary Jim Mattis and State Department officials have long fretted about leaving Syria before a peace agreement can be reached.

ISIS is also widely expected to revert to guerilla tactics once it no longer holds territory. The US has not ruled out that ISIS' leader Abu Bakr al-Baghdadi could still be alive.

A US withdrawal could open Mr Trump up to criticism if ISIS re-emerged.

Mr Trump had lambasted Mr Obama for the withdrawal of US forces from Iraq that preceded an unravelling of the Iraqi armed forces. Iraqi forces collapsed in the face of ISIS' advance into Iraq in 2014.

A senior administration official rejected the comparison to Iraq, where the US had many more troops and would have stayed if Baghdad had provided legal protections for the American forces.

"That's an apples and oranges comparison, given the scope and scale of our engagement in Iraq," the official said.

LAST 1 PER CENT

ISIS declared its so-called "caliphate" in 2014 after seizing large swathes of Syria and Iraq. The hardline 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 square kilometers of territory, with about eight million people under ISIS control. It had estimated revenues of nearly one billion dollars a year.

A senior US official last week said the group was down to its last 1 per cent of the territory it once held. It 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, ISIS will control a diminishing strip of territory along the eastern bank of the Euphrates River in the area where US-backed operations are focused. Militants also control some desert terrain west of the river in territory otherwise controlled by the Damascus government and its allies.

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

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