Tracking British jihadists 'top priority' - Sir Peter Fahy
21 June 2014 Last updated at 13:01 BST
UK police are trying to get a recruitment video for jihadist group Isis - featuring a British student - taken off-line.
In the film, would-be medical student Nasser Muthana, 20, from Cardiff, urges others to fight in Syria and Iraq.
Sir Peter Fahy, counter-terrorism chief for the Association of Chief Police Officers, told the BBC that tracking British jihadists was the "top priority" for the police and security services.
Speaking on BBC Breakfast, he also urged people to come forward if they had suspicions that someone they knew had left the UK to fight.