Britain was on Monday examining an Islamic State video showing a young boy in military fatigues and an older masked militant who both spoke with British accents.

The propaganda video, which could not be independently verified, also shows the killing of five men accused of spying for the West.

The masked man threatens Prime Minister David Cameron and vows that Islamic State will one day occupy Britain before shooting one of the alleged spies in the head.

After the killings of the five men, a young English-speaking boy, who is wearing a black bandanna and appears to be about four or five years old, is shown saying: "So go kill the kuffar right over there".

The US 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 criticising 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.

Britain's Foreign Office said it was aware of the video and was studying its contents.

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