LONDON (AFP) - Police in Britain are "urgently investigating" a video apparently showing a British Islamic State (IS) group fighter railing against Prime Minister David Cameron over air strikes in Iraq, Downing Street said Saturday.

Speaking with a British accent and with his face uncovered, the man, identified by the Site intelligence group as Abu Saeed al-Britani, condemns Mr Cameron and the United States for launching air strikes on IS extremists.

He also urges British Muslims to launch attacks in Britain.

The video was released on Friday, around the time that the IS group put out a video claiming the beheading of British aid worker Alan Henning.

Downing Street said Mr Cameron had raised the video of the fighter at a meeting with security chiefs, police and military to discuss Mr Henning's murder on Saturday.

"At the meeting, the Prime Minister also raised a second video released last night which shows an apparent British ISIL (another term for IS) fighter unmasked delivering a further message," Downing Street said in a statement.

"The police are urgently investigating the contents of the video, including possible terrorism offences relating to it."

Britain's House of Commons voted last week to approve British forces joining US-led raids on targets in Iraq but not Syria.

The country now has eight Tornado jets flying out of Cyprus on combat operations in Iraq.