An Islamic State video showing a young boy in military fatigues and an older masked militant who both spoke with British accents is "desperate" propaganda from an organization that is losing ground, Prime Minister David Cameron said on Monday, December 4, Reuters reports.

The video, which could not be independently verified, also shows the killing of five men accused of spying for Britain. The masked man threatens Cameron and vows that Islamic State will one day occupy Britain before shooting one of the alleged spies in the head.

The footage revived memories of "Jihadi John", a British Islamic State member who appeared in several videos in which hostages were killed before his own death was reported in an air strike late last year.

"It's desperate stuff from an organization that really does do the most utterly despicable and ghastly acts, and people can see that again today," Cameron told reporters.

"This is an organization that's losing territory, it's losing ground ... Britain will never be cowed by this sort of terrorism, our values are so much stronger than theirs. It may take a very long time but they will be defeated."

"This is a message to David Cameron, O slave of the White House, O mule of the Jews," the man said in the 10-minute video released on Sunday.

"How strange it is that a leader of a small island threatens us with a handful of planes. One would have thought you'd have learned the lessons of your pathetic master in Washington and his failed campaign against the Islamic State," the man said.