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 radicalized,” 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 prioritize 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 behavior of perhaps their returned friend or family member.”

Cressida Dick, assistant commissioner of London’s Metropolitan Police, said authorities were “alert to the fact that people may come back and they may have military training.”

“They may seek — although it’s very difficult — to smuggle weaponry here and they may seek to cause violence or to encourage others to cause violence,” she told the BBC.

Prime Minister David Cameron’s office said Monday the government was pressing YouTube and other online services to remove the video and others like it, as officials warned publicity over the recruitment videos could encourage more young British Muslims to join the fight.

ISIL began by fighting against the Syrian regime of Bashar Assad, but it is now leading Sunni militants making major advances in neighboring 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 hometown of Cardiff, 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.

The third Briton appearing in the video was on Monday identified as Abdul Raqib Amin from Aberdeen in Scotland, according to British media reports.