PM hosts emergency meeting after militants post video claiming to show American's beheading and saying Briton would be next

David Cameron will set out the British government's response to the threat to the life of a British man by Islamic State (Isis) at prime minister's questions after an emergency meeting of Cobra on Wednesday morning.

Government sources said Cameron had known of the threat to the man's life for many months so it will not change his short-term calculations, but the revelation of the threat to the public in the gruesome Isis video is likely to put more pressure on the UK to join the air strikes that the Americans have been conducting for weeks against Isis in northern Iraq.

Cameron has already told MPs he regards Isis as a threat to the British way of life, and has not ruled out RAF involvement in air strikes.

Cameron is certain to confer with the US president, Barack Obama, at the Nato summit in Wales on Thursday, both about the threat to further hostages and the wider strategy to combat Isis.

Obama is under intense domestic pressure to be more decisive and is considering whether to extend the air strikes to Syria, the original base of Isis.

Cameron, at the G8 and G20, has led the calls on governments not to pay ransoms to hostages, and he is unlikely to change that policy in this case.

The focus at the Cobra meeting will instead be on identifying the locations and identity of the hostage-takers

Islamic militants released a video on Tuesday that purported to show the beheading of a second captive American journalist, Steven Sotloff, which ended with a chilling warning that a British hostage would be the next to die.

In a video entitled A Second Message to America a masked man is shown carrying out the decapitation of Sotloff, whose life had earlier been threatened in a film that showed the murder of another American journalist, James Foley.

The video, released by Isis on Tuesday, features a voiceover delivered by a British jihadi with a London accent, apparently the same man who was filmed beheading Foley two weeks ago. It ends with the killer threatening another hostage, identified as a Briton.

Screengrab from a video purportedly showing US journalist Steven Sotloff next to a masked Isis fighter in a still image from video released by Isis. Photograph: Reuters

Jabbing a serrated knife towards the camera, the masked man is shown declaring: "I'm back, Obama."

US officials said intelligence experts had begun work on establishing the authenticity of the video, which runs for two minutes and 50 seconds. Cameron condemned the killing as a "despicable act". Sotloff's family issued a statement saying it believed he had been killed.

State department spokeswoman Jen Psaki said the US had seen "reports of a video that purports to be of the murder" of Sotloff.

"The intelligence community will work as quickly as possible to determine its authenticity," she said. "If the video is genuine, we are sickened by this brutal act taking the life of another innocent American citizen. Our hearts go out to the Sotloff family."

In the video, seen by the Guardian, Sotloff appears on his knees – his hands tied behind his back – beside a man holding a serrated knife, reminiscent of those shown in the Foley video.

The masked fighter indicated he was the same man who murdered Foley, saying: "I'm back, Obama, and I'm back because of your arrogant foreign policy toward the Islamic State."

The video was recorded in desolate desert landscape that resembled the location in which Foley was murdered, although the latest video showed a flatter terrain. It is unclear when it was made, but it appears to be recent: the masked killer makes reference to recent US bombings in Iraq, including the air strikes near Amerli that began on Saturday. Sotloff's hair and his beard are also longer than in the Foley video, in which Sotloff also appeared.

In the latest video, Sotloff is dressed in an orange jumpsuit, apparently mimicking those used at Guantánamo Bay, and is made to deliver a statement blaming Obama for his murder. The killer states: "You, Obama, have yet again, through your actions, killed yet another American citizen." He then appears to slice Sotloff's throat.

The black-masked killer again urges Obama to cease his bombing campaign in Iraq, and says that if the US does not cease bombing, "our knife will continue to strike the necks of your people".

The warning is extended to governments that might join the "evil" actions of the US against Isis. It is around this point that the video cuts to another kneeling captive, stated to be a British national.

British intelligence officials have been working urgently to try to identify Foley's killer, reportedly using sophisticated voice recognition techniques to help narrow down a list of potential British jihadi suspects.

UK government sources said that an early analysis of the video suggested that the British man whose voice issued this threat was the same man heard on the video that depicted the murder of Foley.

Sotloff, 31, a freelance journalist from Miami, Florida, disappeared near the Syrian-Turkish border in August last year while working for the Time and Foreign Policy magazines.

A spokesman for Sotloff's family said: "The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time." Last week, Sotloff's mother Shirley had pleaded with Abu Bakr al-Baghdadi, the leader of Islamic State, for the release of her son, whom she described as an innocent journalist.

She recorded the plea after the video of Foley's killing last month was released. In that film, Foley's killer said her son would be next in line if the militant group's demands were not met.

Tuesday's video followed a similar pattern. It finished with footage of the British hostage, in the same style of orange jumpsuit that both Foley and Sotloff were wearing, suggesting he was their next intended victim.

Finally, with the Briton still in shot, the masked killer states: "Back off and leave our people alone." A caption identifies the man, although the Foreign Office in London requested that his name not be published.