David Cameron has dismissed the latest Daesh execution video as "desperate stuff" from a group that is "losing territory".

The U.K. Prime Minister was responding to gruesome footage of a masked man speaking in a British accent who taunts him before executing the prisoners kneeling in front him.

"It's desperate stuff from an organization that really does do the most utterly despicable and ghastly acts and people can see that again today," Cameron said in comments to broadcasters during a visit to East London.

"But this is an organization that's losing territory; it's losing ground [and] increasingly losing anybody's sympathy, and this again shows what an appalling organization we're up against.

"They hate us not for what we do but for what we are-the fact that we are a successful, tolerant, democratic, multi-faith, multi-ethnic nation."

He continued: "But I know that 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 video which was posted online late on Sunday night, the masked man waves a handgun as he describes Cameron as "the leader of a small island [who] threatens us with a handful of planes" and "an imbecile".

U.K. security services have launched an investigation aimed at identifying the man, who appears to be emulating the style of Mohammed Emwazi, the 27-year-old Daesh fighter from London known in the tabloid press as 'Jihadi John'.

Emwazi was killed in a U.K. drone strike last November. Britain began regular air strikes against Daesh in Syria the following month.

In Sunday's video, the extremist and four other masked men are shown shooting dead five kneeling prisoners, believed to be Syrians, who are accused of spying for the U.K.