America’s Kurdish allies in the fight against Isis have condemned Donald Trump’s decision to withdraw US forces from Syria, saying it will destabilise the region.

Over the past four years, and with the backing of the US, the Kurdish-led Syrian Democratic Forces (SDF) have captured swathes of territory in the country’s north and east from the extremist group.

But on Wednesday, Mr Trump ordered a full withdrawal of American forces from Syria, a dramatic turnaround in US policy that caught its allies by surprise.

“The White House decision to withdraw from the north and east of Syria would negatively impact the campaign to fight terrorism, and it would give terrorism and its supporters military field and political clout to revive itself again,” the SDF said in a statement on Thursday.

The group added that the fight against Isis “has not ended and the group has not been defeated”, and that the fight against the group was at a “critical” stage that required more support from the US-led coalition.

The US has more than 2,000 troops in Syria to support local Kurdish forces in the battle against Isis. The town of Hajin, on the banks of the Euphrates river in eastern Syria, is Isis’s last major stronghold in Syria. After three months of intense fighting, the SDF announced on Friday that it had taken most of the town. Beyond Hajin lies a string of smaller villages still held by Isis, where an estimated 5,000 fighters remain.

But even as that fight continues, Mr Trump was said to be eager to declare victory.

“They’re all coming back and they’re coming back now. We won,” Mr Trump declared on Wednesday in a video posted on Twitter.

He had earlier declared: “We have defeated Isis in Syria, my only reason for being there during the Trump presidency.”

The order to withdraw also signifies an end to the US air campaign against Isis, according to US officials. This had not necessarily been the case, given that unlike ground troops air assets like fighter jets are not based in Syria but fly in from nearby nations.

The US-led air assault has been a major part of the plan to push back Isis in both Syria and Iraq – with more than 100,000 bombs and missiles having been used on targets in the two nations since 2015.

Early on Thursday Mr Trump said that his decision to pull out troops should be “no surprise” in a number of tweets.

He said: “​Does the USA want to be the Policeman of the Middle East, getting NOTHING but spending precious lives and trillions of dollars protecting others who, in almost all cases, do not appreciate what we are doing?”

That statements from Mr Trump appear to stand in stark contrast to almost every top US defence official over the past year, who have gone to great lengths to stress that Isis will not be defeated when it loses the last of its territory, but will instead revert back into a deadly insurgent group with the ability to carry out attacks in Iraq and Syria, and further afield.

Last month, James Jeffrey, the US special representative for Syria, said that “the enduring defeat means not simply smashing the last of Isis’s conventional military units holding terrain, but ensuring that Isis doesn’t immediately come back in sleeper cells, come back as an insurgent movement”.

Tobias Ellwood, a UK defence minister said he “strongly disagreed” with Mr Trump’s statement, adding: “It [Isis] has morphed into other forms of extremism and the threat is very much alive.”

One official told Reuters that the pullout could take up to 100 days, and the White House indicated that is had already begun.

Perhaps of even greater concern than the fight against Isis for the SDF is the threat of a Turkish attack.

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The alliance the US and SDF has angered Turkey, a Nato ally, which views the Kurdish forces that fight in the SDF as terrorists. Turkey’s President Recep Tayyip Erdogan vowed last week to launch a military operation into Syria against the group. The presence of US forces alongside the Kurdish fighters was seen as a bulwark against an attack.

Kurdish fighters have already begun preparations for a Turkish attack by digging trenches close to the border. Speaking on Thursday, Turkey’s defence minister reiterated Ankara’s threat.