An Islamic State 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 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.

"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 Daesh and what the world faces with these terrorists. It is also clearly a propaganda tool and should be treated as such," the spokeswoman said, referring to Islamic State by one of its Arabic acronyms.

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 saying: "So go kill the kuffar [unbelievers] right over there".

The United States said in November it had killed Mohammed Emwazi, a British citizen dubbed Jihadi John who became a symbol of Islamic State 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 criticizing Cameron.

"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.

In November, British officials said that up to 800 Britons had traveled to Iraq and Syria, some to join Islamic State. About 50 percent had returned home while about 70 were believed to have been killed.