Video report by ITV News Washington correspondent Robert Moore
Donald Trump's claim that so-called Islamic State has been defeated in Syria has been widely rejected.
The US president announced on Wednesday that he was pulling all 2,000 US ground troops out of Syria after declaring a "historic victory" against the group.
But senior members of his own party and the international community questioned the move.
Senator Lindsey Graham, often an avid support of Mr Trump, called the decision “a disaster in the making”, while UK defence minister Tobias Ellwood said the threat posed by Islamic extremism in the region was "very much alive".
The decision was welcomed by Moscow, with Russian foreign ministry spokeswoman Maria Zakharova saying it could result in "genuine, real prospects for a political settlement" in Syria.
The US began airstrikes in Syria in 2014, and ground troops moved in the following year to battle IS and train Syrian rebels in a country torn apart by civil war.
On Wednesday, Mr Trump abruptly declared their mission accomplished in a tweet, writing: "We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency."
He later posted a video on Twitter in which he said it was “heartbreaking” to have to write letters and make calls to the loved ones of those killed in battle.
“Now it’s time for our troops to come back home,” said Mr Trump, who has argued for the withdrawal since he was a presidential candidate.
The US will still maintain a military presence in the region, in particular air power.
The decision will fulfil Mr Trump’s long-stated goal of bringing troops home from Syria, but military leaders argue that IS remains a threat and could regroup as it battles in Syria’s long-running civil war.
The UK has formed part of the US-led coalition carrying out air strikes against IS in Syria.
Responding to Mr Trump's tweet, Mr Ellwood disagreed and said the threat "has morphed into other forms of extremism and the threat is very much alive".
A UK Government spokesman said the threat posed by IS, which it calls Daesh, still remained.
The spokesman said the campaign has made "huge progress" but added: "Much remains to be done and we must not lose sight of the threat they pose. Even without territory, Daesh will remain a threat.
"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.
"We remain committed to the Global Coalition and the campaign to deny Daesh territory and ensure its enduring defeat, working alongside our critical regional partners in Syria and beyond.
"As the situation on the ground develops, we will continue to discuss how we achieve these aims with our Coalition partners, including the US.
"This Government will continue to do what is necessary to protect the British people and our allies and partners."
Israeli Prime Minister Benjamin Netanyahu, who remains concerned about Iranian efforts in the area, reacted in non-committal fashion after talking with Mr Trump by telephone.
“This is, of course, an American decision,” he said.
Israel will learn of the timetable and manner of withdrawal, he said, and no matter what “we will safeguard the security of Israel and protect ourselves from this arena”.
Leading Republican senators reacted with displeasure to the news.
Senator Lindsey Graham said the biggest winners from the decision would be IS and Iran.
He said: "Withdrawing our forces is basically tearing down the best defence we have on the homeland.
"The day we withdraw from Syria puts in motion, I think, a lot of bad things in the region and we'll eventually pay for it here, just as we did in Iraq."
Marco Rubio of Florida said the withdrawal would be a “grave error with broader implications” beyond the fight against IS, saying it further strengthens the Russian and Chinese argument that America is an unreliable ally.
Just last week, the US special envoy to the anti-Isis coalition, Brett McGurk, said US troops would remain in Syria even after IS was driven from its strongholds.
“I think it’s fair to say Americans will remain on the ground after the physical defeat of the caliphate, until we have the pieces in place to ensure that that defeat is enduring,” Mr McGurk told reporters on December 11.
“Nobody is declaring a mission accomplished. Defeating a physical caliphate is one phase of a much longer-term campaign.”
The withdrawal decision, however, is likely to be viewed positively by US ally Turkey, and comes following several conversations between Mr Trump and Turkish President Recep Tayyip Erdogan over the past weeks.
The Turks have targeted US-backed Kurdish troops along the Syria-Turkey border, which Turkey considers an insurgent threat.
A US withdrawal – including the end of joint US and Turkish patrols along the border – could open the door for more Turkish operations against the Syrian rebels.