The Syrian Democratic Forces (SDF) are expected to announce a regional defeat over IS in the coming days after a four-and-a-half-year battle against the group covering Syria and Iraq, the Daily Telegraph newspaper reported.
The United States has not made any alternative plans to support SDF to fight IS after announcing the withdrawal of about 2,000 US special forces by April.
“President Trump’s decision to withdraw so quickly is as bizarre as it is short-sighted,” Bob Seely, member of the Foreign Affairs Select Committee and co-chair of all-party parliamentary group Friends of Syria, told the Telegraph.
“We need to be realistic but supportive. Syrian Kurdish territory is part of Syria. However, we, the US and France need to be willing to repay that debt of thanks to the Kurds and make sure that in Syria they are not left to the mercy of (IS) or the Assad government,” the Conservative MP said.
“If we are not willing to stand by our allies, we will find we have fewer of them.”
Officials from the SDF, which is thought to have lost as many as 10,000 fighters battling IS, said that they had been left feeling abandoned.
“I’m worried about the US withdrawal, we fought together and I don’t believe it was a wise decision,” said Commander Adnan Afrin. “As I see it, it’s a decision to leave halfway through.”
Once the "caliphate" is declared over, Commander Afrin said his forces will face another fight against IS’ toxic ideology.
Jeremy Hunt, Foreign Secretary, on Monday said the world should not “mistake territorial defeat for final defeat” and that forces fighting IS should not claim “victory too quickly”.
A Pentagon report published this month warned that IS "could likely resurge in Syria within six to 12 months and regain limited territory" if sustained pressure is not maintained.
The jihadist group retains a presence in eastern Syria's vast Badia desert as well as remote territory in western Iraq, and has continued to claim deadly attacks in SDF-held areas.
While IS will soon no longer have fixed positions anywhere in Iraq or Syria, its surviving fighters have reverted to guerrilla warfare and remain a potent force.
The jihadists maintain sleeper cells along the border with Iraq, as well as in cities they once ruled, and have carried out a number of IED and suicide attacks.
The SDF arrested 63 suspected militants in IS' former Syrian capital Raqqa, which fell more than a year ago, last week.
Kurdish leader (Elham Ahmed) spent the past two weeks in Washington meeting with US officials in an effort to secure assurances they would be safe.
There is also the unresolved issue of the thousands of foreign IS suspects and their families being held by the SDF.
The UK and a number of other countries have so far resisted calls to repatriate their nationals, despite warnings from the SDF that they have neither the space or resources to hold them forever.