Militant Video via The Associated Press
In this undated image taken from video posted online by Communications Arm of Islamic State, circulating online Sunday Jan. 3, 2016, purporting to show a member of the Islamic State group brandishing a gun and talking to camera, before Islamic State group members seem to execute five men they accuse of spying for Britain in Syria. The video could not be independently verified, but the footage Sunday bears the markings of IS media wing.

LONDON — British security officials are studying an Islamic State group video that threatens more action against the United Kingdom.

Officials are looking for clues Monday about the identity of a masked man who speaks with a British accent on the video before shooting a purported British spy in the head. Four others accused of spying for Britain are also shot dead by other masked figures who do not speak.

The masked man, occasionally pointing a gun at the camera for emphasis, vows the extremists will soon invade Britain and establish Shariah law.

The anti-British propaganda video is the first to surface from the Islamic State extremists since Britain's Parliament approved Cameron's December request for authorization to launch airstrikes against IS positions in Syria. The masked figure characterizes Prime Minister David Cameron as an "imbecile."

Cameron's spokeswoman, Helen Bower, would not say whether Cameron had seen the video. She said British intelligence services were working to identify the masked militant and a young child who appears in the video.

Bower said the video is "propaganda . and should be treated as such."

Bower declined to comment on the allegation that the five men who were killed were British spies.

The 10-minute video concludes with footage of a young boy of about 4 or 5 who threatens Britons.

The extremists frequently use children in propaganda videos.

"It shows that the new generation is already part of the fight," said Olivier Guitta, director of the consulting group GlabalStrat,

The video circulated Sunday features an apparent IS replacement for Mohammed Emwazi, the British-accented man known as "Jihadi John," who was killed in a drone strike in Syria in November. He had figured prominently in earlier IS propaganda videos.

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Associated Press writer Jill Lawless contributed.