The possibility of militants returning was "a matter of great concern", Border Security and Asylum Minister Angela Eagle said.

Britain along with other European countries including Germany and France on Monday said they would freeze all pending asylum requests from Syrians.

Around a dozen "hardened Islamic State fighters" from the UK were being held in prisons in northeastern Syria which is controlled by Kurdish forces, the Daily Telegraph reported.

Security sources told the newspaper Britain was at risk from the possible return of such fighters if the prisons, run by the Kurdish authorities, were to be affected by the chaos in the country.

"Rest assured that the intelligence services are keeping a very close eye on what's going on and we're in contact with all of our allies to see how this pans out," Eagle said.

Assad was ousted after the Islamist-insurgent group Hayat Tahrir al-Sham (HTS) swept from their bastion in northwestern Syria through government-held territory in northern and central Syria, reaching the capital Damascus in a lightning offensive.

In 2014, the so-called Islamic State (IS) group declared a "caliphate" in large parts of Syria and neighbouring Iraq, implementing its brutal rule on millions.

Hundreds of Britons are estimated to have travelled to the region to join the caliphate before it collapsed in 2019 following fierce battles with US-backed Kurdish fighters.

But pockets of IS militants remain in their hideouts in Syria.

In January 2022, IS fighters stormed a jail in the Kurdish-controlled city of Hasakeh, with dozens of IS members believed to have escaped in the early hours of the attack.