London: An IS militant group video showing a young boy in military fatigues and an older masked militant who both spoke with British accents should be viewed as a propaganda tool, Prime Minister David Cameron said on Monday.
The video, which could not be independently verified, also shows the killing of five men accused of spying for the West.
The masked man threatens Cameron and vows that IS will one day occupy Britain before shooting one of the alleged spies in the head.
The footage revived memories of a militant who was a British IS member and appeared in several videos in which hostages were killed before his own death was reported in an air strike late last year.
"We are examining the content of the video and the prime minister is being kept updated on that," Cameron's spokeswoman said of the latest footage. She was not aware whether Cameron himself had watched it.
"It serves as a reminder of the barbarity of IS and what the world faces with these terrorists. It is also clearly a propaganda tool and should be treated as such," the spokeswoman said.
When asked whether the men shown had been spies, the spokeswoman declined to comment on intelligence matters but said the group's past propaganda had not all been true.
After the killings of the five men, a young English-speaking boy, who is wearing a black bandana and appears to be about four or five years old, is shown.
The United States said in November it had killed Mohammed Emwazi, a British citizen who became a symbol of IS brutality after appearing in several hostage execution videos.
The masked militant shown in the new video was different from Emwazi but spoke in a clear English accent, waving a gun at the camera while criticising Cameron.
"This is a message to David Cameron," the man said in the 10-minute video released on Sunday.
In November, British officials said that up to 800 Britons had travelled to Iraq and Syria, some to join IS. About 50 per cent had returned home while about 70 were believed to have been killed.