American soldiers stationed in Syria will soon be returning home as the US rapidly pulls out all its 2000 troops from the previous Islamic State stronghold within the next 24 hours.

The withdrawal is a major departure from previously stated US policy and the would fulfill a the goal of Syria, Iran and Russia, and risks diminishing US influence in the region, a US defence official was quoted as saying by CNN.

White House spokeswoman Sarah Sanders made the announcement on Thursday morning (Australia time) after President Donald Trump declared that ISIS had officially been defeated.

“We have defeated ISIS in Syria, my only reason for being there,” Mr Trump wrote on Twitter.

The President’s comments comes in the face of recent assurances from the US-led coalition, denying any change to the US presence in Syria.

Ms Sanders said while the US had begun returning troops home, she suggested that the country would still remain somewhat engaged as it transitioned to the next phase of its four-year campaign against IS.

“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.

There are fears however that a complete withdrawal of US troops, particularly in the Kurdish region in northern Syria, where ISIS once maintained a stronghold, will allow the terrorist group to regain control over the territory. 

Mr Trump’s decision to pull out all military personnel came after a phone call he had with Turkish counterpart Tayyip Erdogan on Friday.

“Everything that has followed is implementing the agreement that was made in that call,” a US official said.

There are additional fears that Mr Trump’s decision would upend assumptions about a longer-term US military presence in Syria, which senior US officials have advocated to help ensure IS cannot re-emerge.

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.

Reports of a full US military withdrawal drew immediate criticism, including from some of Mr Trump’s fellow Republicans.

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.

One official told Reuters that partners and allies had been consulted. 

A complete withdrawal of US troops from Syria would still leave 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.

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

-with AAP