Intelligence services say Isis claims about identity of five victims are false and that it is likely they were working for Syrian resistance

Britain’s intelligence agencies have made a preliminary assessment that the Islamic State made false claims that the five hostages murdered in a new propaganda video had acted as UK spies.

David Cameron, who confirmed that he had viewed part of the footage, said the video was “desperate stuff” from an organisation that is losing territory under pressure from allied airstrikes.

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Britain’s three intelligence agencies – MI5, MI6 and GCHQ – have been analysing the video since its release over the weekend and concluded that none of the hostages were working for Britain.

The agencies cannot definitively rule out that they may have been working for other allied countries. Their strong assumption, however, is that the hostages were working for Syrian resistance opposed to Isis in their territory. A vital part of the resistance’s work is the dissemination of video to highlight the horrifying impact of life under Isis.

But Hamoud al Mousa, who describes himself as the founder of the anti-Isis activist group Raqqa is Being Silently Slaughtered, told the Guardian he did not know the victims and that they were not part of the organisation.

Downing Street declined to comment on intelligence matters. But the prime minister’s spokeswoman cast doubt on claims in the video that the apparent victims were UK spies. “This does appear to be a propaganda tool and not all of Isil’s propaganda in the past has been true,” the spokeswoman said.

Cameron said the video showed that Isis was now on the back foot. “I watched a part of [the video] yesterday and I think it is absolutely desperate tactics from an organisation that is clearly now on the back foot,” he told LBC radio.

“It’s losing territory. It lost Ramadi, but what is so clear about it is that they continue to use the most disgusting and repulsive tactics and actions which demonstrate exactly who these people are and I think that’s plain for everyone to see.

“Any normal person watching that video will realise that describing these people as a death cult and as truly repulsive is utterly fair comment and so I don’t mind people seeing, within limits, a little bit of this just to remind them about what a truly ghastly, murderous organisation we’re up against.”

No 10 illustrated its claim that Isis is facing severe pressure by saying that RAF aircraft had taken part in 11 airstrikes against Isis targets in Syria since parliament approved the extension of the military operation from Iraq. These included the targeting of Isis targets south of its Syrian base of Raqqa on Christmas Day. The spokeswoman said that RAF planes had been focused in recent days on efforts to expel Isis from the Iraqi city of Ramadi.

Ministers fear that the attacks in Paris and the downing of a Russian aircraft over the Sinai peninsula show that Isis will resort to al-Qaida’s tactics of attacks in the west as it struggles to establish a caliphate.

There will be some scepticism over the claims in Whitehall that Isis is on the back foot. Iraqi forces made premature claims about the recapture of Ramadi, while Baghdad will also face a lengthy and bloody battle to retake Mosul and Fallujah. Progress against Isis in Syria is also slow.

The claims about Isis came after the release of the 11-minute Isis video which includes a “message for David Cameron”, read by a masked man with a British-sounding accent who threatens attacks in the UK.

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Scotland Yard’s counter-terrorism command and MI5 and other intelligence agencies were investigating the identities of those featured in the video. Police refused to confirm or deny media reports about the identity of the child who features at the end of the video.

Social services in Lewisham said they were working with police investigating the identity of the child, who speaks in English in the video.

Security officials believe the threats in the video issued against the UK show Isis’ continued desire to attack Britain and the terrorist threat levels remains at severe, meaning an attack is assessed to be highly likely.

The UK security services are searching a pool of about 300 Britons known to have travelled to Syria and Iraq since 2012 to join terrorist-related activities and who are still active. The services are aware of about 800 people in total who have travelled to the region but about 400 of these have since returned to the UK and another 50 to 100 have been killed in fighting.

Of the 300 or so who have remained, the numbers can be narrowed further as this includes a small but significant number of women. Some also went to fight not for Isis but for other groups such as al-Nusra Front.