ISIS continues to pose a significant threat almost four years after its last holdouts in Syria were defeated, according to experts.
They also warned the terror cell is far from extinct and will seek to bring 10,000 of its fighters from Syrian prisons and detention camps next year.
At its strongest point, the terror group controlled a third of Syria and 40 per cent of Iraq, following its rise in mid-2014.
But the success of the US coalition in ousting the group from towns and cities, including its capital Raqqa in Syria, meant that by March 2019 its physical caliphate had fallen.
Dr Shiraz Maher, director of the International Centre for the Study of Radicalisation and a member of the war studies department at King's College London, said such a plot would pose the "single greatest security threat to the West".
"Blink and you'll miss it and suddenly ISIS will be back," he told Sky News.
"The Syrian Democratic Forces have repeatedly said that this is a ticking time bomb that they are sitting on (that) they are not capable of dealing with themselves alone.”
Iraq police protect farmers from ISIS - in pictures
There are fears that a possible ground offensive by Turkey in Syria could provide the perfect environment for ISIS to once again take control of vast swathes of territory.
Matthew Henman, a terror expert at intelligence firm Janes, told Sky News that ISIS “has maintained a steady tempo of insurgent violence” since being stripped of its land.
The Islamic State in Khorasan Province, an affiliate of ISIS, has increased its attacks since the Taliban regained power in summer 2021.
The Kurdish-led SDF is backed by the US and its allies, and has served as a key partner in the West’s fight against ISIS.
Western nations have been repeatedly criticised for failing to bring back citizens who joined ISIS from camps in Syria and Iraq.
While many analysts doubt a return of ISIS would see it rebuild its caliphate in its previous form, the terror group has in recent years been mobilising thousands of remaining fighters.
At the height of its control, ISIS held territory stretching from the outskirts of Baghdad to western Syria and encompassed more than 10 million people.
Western leaders will also fear that a strengthening of the group will once again embolden lone wolves or terror cells in their nations to carry out domestic attacks.
The SDF on Monday said an attack by ISIS militants in Raqqa killed six of its troops.
SDF commander Mazloum Abdi said in a statement that an ISIS cell had targeted security and military buildings in the city, killing six of his fighters and wounding an unspecified number of others.
He added that intelligence gathered by the group “indicates serious preparations by (IS) cells.”
Siamand Ali, a spokesman for the Kurdish-led Syrian Democratic Forces, told The Associated Press that a group of five people believed to be part of an ISIS sleeper cell, two of them wearing explosive belts, had attacked checkpoints and guard points of Raqqa’s Internal Security Forces.
During the ensuing clashes, he said, one of the attackers was killed and another arrested. SDF and Internal Security Forces units are searching for the remaining attackers.