An estimated 500 British people are currently fighting in Syria, but the number could be far higher, the UK's counter-terrorism chief for the Association of Chief Police Officers told ITV News.
Sir Peter Fahy said many British families reported that young men in their family have gone abroad, but are not sure where.
"We know that a number of people have gone to areas like Turkey, a number of families have reported that the young men in the family have gone abroad, they're not sure where," he said.
Leaders of a Cardiff mosque, attended by 3 young men who are now fighting in Syria, say they had nothing to do with their radicalisation.
The Home Secretary has told ITV News that young women have left Britain to fight for the extremists group ISIS in Syria and Iraq.