LONDON • An Islamic State in Iraq and Syria (ISIS) video showing a young boy in military fatigues and an older masked militant who both spoke with a British accent is desperate propaganda from an organisation that is losing ground, Prime Minister David Cameron said.

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

The masked man threatens Mr Cameron, and vows that ISIS will one day occupy Britain, before shooting one of the alleged spies in the head.

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

"It is desperate stuff from an organisation that really does do the most utterly despicable and ghastly acts, and people can see that again today," Mr Cameron told reporters on Monday. "This is an organisation that is losing territory, it is 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."

In the latest fighting in Iraq, ISIS has largely been pushed out of the city of Ramadi, its biggest prize of last year.

The United States said in November that it had killed Mohammed Emwazi who, as "Jihadi John", had become an ISIS symbol.

The voice and appearance of the masked militant shown in the new video was different from Emwazi's, but he spoke in a clear English accent, waving a gun at the camera while criticising Mr 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 would have learnt the lessons of your pathetic master in Washington and his failed campaign against the Islamic State," the man said, using another name for ISIS.

British media speculated that the militant might be Siddhartha Dhar, who is also known as Abu Rumaysah, a convert from Hinduism and a high-profile Islamist, although security experts were divided on whether it was him.

Dhar left Britain with his family to travel to Syria despite being on police bail after being arrested in late 2014 on suspicion of being a member of a banned organisation.

After the killings of the five men in the video, a young English-speaking boy, who appears to be about four or five and wearing a black bandana, is shown saying: "So, go kill the kuffar (non-believers) right over there."

The father of Ms Grace Dare, a woman from London who left Britain to join ISIS and marry a militant, said he believed the boy was her son. "It is my grandson. I can't disown him," Mr Sunday Dare told Channel 4 News. "He doesn't like it over there. It is propaganda. They are just using a small boy... They are just using him as a shield."

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

REUTERS