At least 60 former Britons who joined the terror group, including "Jihadi Jack" and Shamima Begum, are among 45,000 men, women and children held at the al-Hol and Roj camps in north-east Syria.
Of these, around 10,000 are thought to be battle-hardened fighters.
Intelligence chiefs have warned they pose a significant threat as it is feared they will be released by the new regime after dictator Bashar al-Assad was deposed last weekend.
To stop IS extremists heading for Britain to potentially carry out terror attacks, the RAF will deploy RC-135 surveillance aircraft and "spy in the sky" Reaper drones.
Their advanced image technology and voice recognition systems can identify and track high value targets from 50,000ft.
Among former British citizens detained at the camps are Jack Letts, 29, from Oxford, who was nicknamed "Jihadi Jack" after travelling to Syria in 2014, and Begum, 25.
She was a 15-year-old schoolgirl when she travelled from Bethnal Green, East London, to become an IS bride in 2015.
Beleaguered Kurdish troops of the Syrian Democratic Forces guarding these detainees have warned unless they get support, the extremists will break out and rise again.
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