Syria and Iraq: Tracking British jihadists 'a top MI5 priority'
20 June 2014 Last updated at 22:26 BST
Tracking British jihadists who are fighting with extremist groups is now the top priority for the British security services.
Footage has emerged online which appears to show British and Australian fighters with Isis, urging Muslims to join them in Syria and Iraq, as part of an online social media strategy.
Meanwhile the BBC has learned that MI5 are devoting the greatest amount of their casework to tracking jihadists leaving the UK for Syria - and returning.
Security correspondent Frank Gardner reports.