Richard Barrett, ex-head of counter-terrorism at the MI6 overseas security agency, said 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 Islamic State of Iraq and the Levant (ISIL) terrorist group.

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.

Thousands of foreigners, mainly from Arab and European countries, are said to be operating against the Syrian government.

Western spy agencies have already warned that the foreign militants pose a potential security threat when they return home.

RA/NJF