LONDON (Reuters) - An Islamic State video showing a young boy in military fatigues and an older masked militant who both spoke with British accents is "desperate" propaganda from an organisation that is losing ground, 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 Britain.
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.
"It's desperate stuff from an organisation that really does do the most utterly despicable and ghastly acts, and people can see that again today," Cameron told reporters.
"This is an organisation that's losing territory, it's 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, Islamic State has largely been pushed out of the city of Ramadi, its biggest prize of last year.
The United States said in November it had killed Mohammed Emwazi, who as "Jihadi John" had become a symbol of Islamic State. The voice and appearance of the masked militant shown in the new video was different from Emwazi, but he 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.
Some 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.
Cameron's spokeswoman said Britain was examining the video and the prime minister was being kept updated. 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 executed 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 father of Grace Dare, a woman from London who left Britain to join Islamic State and marry a militant, said he believed the boy was her son.
"It's my grandson. I can't disown him," Sunday Dare told Channel 4 News.
"He doesn't like it over there. It's propaganda. They are just using a small boy. He doesn't know anything. 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 Islamic State. About 50 percent had returned home while about 70 were believed to have been killed.
(Reporting by Guy Faulconbridge, Michael Holden, Kylie MacLellan and Stephen Addison; Editing by Giles Elgood and Pravin Char)