British security forces will find it impossible to monitor all the estimated 500 jihadists fighting in Syria and Iraq after they return home, a former top intelligence official said Monday.
Richard Barrett, ex-head of counter-terrorism at the MI6 overseas security agency, told the BBC that authorities would have to try to identify the biggest threats.
Barrett said the number who had gone to Syria "could be as high as 500 by now".
His comments came after several young British men featured in a YouTube recruitment video for the jihadist group Islamic State of Iraq and the Levant (ISIL).
"The trouble is, you don't know which ones are coming back just wanting to get on with their lives and which ones are coming back quite severely radicalised," he said.
He said it would be an "enormous challenge" and added that there was "absolutely no way" the security services could follow all of them, "that's out of the question."
"Clearly they'll have to prioritise and they'll have to choose those that they think are likely to pose the greatest risk," he added.
"Beyond that I think they'll have to rely very much on members of the community and other people expressing their concern and worry about the behaviour of perhaps their returned friend or family member."
ISIL began by fighting against the Syrian regime of Bashar al-Assad, but it is now leading Sunni militants making major advances in neighbouring Iraq.
The father of two young British fighters -- Nasser Muthana, 20, who appeared in the ISIL video, and Aseel Muthanam 17 -- said they had "betrayed" Britain.
"This is my country. I came here aged 13 from Aden when I was orphaned," father Ahmed Muthana told the Guardian newspaper.
The Daily Mail newspaper said a mosque in their home town of Cardiff, Wales, where the brothers worshipped had played host in 2012 to Saudi cleric Mohammed al-Arifi, who has called for holy war and the overthrow of the Assad regime.