Isis militants may be holding other Western hostages whose lives are now in grave danger following the murder of the American journalist James Foley, it emerged last night.

Reports from America suggested that at least three other US hostages were being held alongside Mr Foley in the months leading up to his death and threats have been made against their lives.

The militants had demanded a ransom of $132.5m (£80m) for Mr Foley’s releaseh, his employer said last night. It went unpaid in line with US policy, which prohibits negotiations with terrorists.

The journalist’s murder and the threat to Western lives has so far not had any effect on US air strikes on Isis positions in Iraq. Since the video of his beheading was released on Tuesday, US aircraft have launched 14 strikes including several yesterday. US Navy fighters and drones also provided air cover for Kurdish and Iraqi forces fighting Isis near the city of Mosul.

In the UK, police and intelligence agencies mounted an intensive operation to identify the masked man who spoke with a British accent on the video before executing Mr Foley. They are understood to be using voice recognition software to trawl police records and extremist material on the internet for a match. A public appeal has also been launched.

Richard Barrett, a former British security official and co-ordinator of the United Nations al-Qa’ida Taliban Monitoring Team, said he expected the man to be identified fairly quickly but that bringing him to justice may prove more difficult. “The most important thing is to demonstrate that you cannot do these things even in the middle of the desert, just because you are in something called Islamic State [as Isis now calls itself] or the caliphate,” he said.

David Cameron has now resumed his holiday in Cornwall. A spokesman for the Prime Minister said he remained “in close contact with his team”.

Dozens of journalists and aid workers are thought to have been taken hostage in Iraq and Syria, but few have been named. One is Steven Sotloff, a 31-year-old American journalist, who was captured near Aleppo in northern Syria in August last year. He is being held by the same group responsible for the death of Mr Foley, who have also threatened to murder him.

Sir Iqbal Sacranie, an adviser to the Muslim Council of Britain, said Mr Foley’s murder showed that the UK must act urgently to stop other young Muslims being seduced by a “sub-culture of this ‘jihadi-cool’.” He added: “This is a problem that affects all of us”. He added that Britain’s Muslim community was pushing the message that acts like this are “totally alien to Islam” and that families were informing authorities when they discovered that their sons had headed to the Middle East to fight.

Haras Rafiq, head of outreach at the counter-extremism group Quilliam, said he was “not surprised” that Mr Foley’s killer had a British accent. “We’ve had decades of Islamist groups like the Muslim Brotherhood indoctrinating our youngsters that they have to struggle for this utopian Islamic caliphate.