British Prime Minister David Cameron on Monday dismissed a new video in which five purported British spies are shot dead by masked Islamic State extremists as propaganda from a group that is losing control of territory in Syria and Iraq.

"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," he said. "But this is an organization that's losing territory, it's losing ground, it's, I think, increasingly losing anybody's sympathy, and this again shows what an appalling organization we're up against."

British security officials are studying the video for clues about the identity of a masked man who speaks with a British accent on the video before shooting a captive in the head. The masked man, occasionally pointing a gun at the camera for emphasis, vows the extremists will soon invade Britain and establish Shariah law.

The masked figure is 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. The British-born Muslim had figured prominently in earlier IS beheading videos.

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 extremist group has recently lost control of the Iraqi city of Ramadi after an Iraqi military assault backed by U.S. airpower.

Cameron's spokeswoman, Helen Bower, said Britain's security forces are scrutinizing the video.

"We are examining the content of the video and the prime minister is being kept updated on that," she said. "It serves as a reminder of the barbarity of Daesh (Islamic State) and what the world faces with these terrorists. It is also clearly a propaganda tool, and should be treated as such."

Asked about the claim the five men executed were British spies, she said: "This does appear to be a propaganda tool and not all of ISIL's propaganda in the past has been true."

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, said Olivier Guitta, director of the consulting group GlabalStrat.

"It shows that the new generation is already part of the fight," he said.