Media playback is unsupported on your device

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.

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.