Peter Kassig
Isis has threatened to kill US hostage Peter Kassig next. Source: Handout

Last night, British aid volunteer Alan Henning became the fourth western hostage to be murdered on camera by the masked Isis militant nicknamed Jihadi John.

At the end of the film another hostage, former US soldier and aid worker Peter Kassig is paraded before the world, and threatened with death unless the US and its western allies cease their air campaign against Isis, in what has now become a sickeningly familiar pattern.

How many western hostages does Islamic State hold? How long can the gruesome series of executions continue?

In September, a US official told CNN that a number of US citizens were being held hostage by militant groups in Syria, though he did not say how many exactly.

Two female Italian aid workers were kidnapped by an unnamed Islamist group in Aleppo in August, along with a Danish national and Japanese national.

A 26-year-old female US aid worker was kidnapped by militants last year, with Islamic State reportedly demanding $6 million (£3.76m) to secure her release.

The International Committee of the Red Cross said a year ago that three of its workers had been kidnapped.

US freelance journalist Austin Tice vanished in Syria in August 2012. A short video of him blindfolded with bound hands was released on YouTube shortly after, but nothing has been heard from him or his captors since.

British hostage John Cantllie has recorded a series of propaganda videos for the group, presumably under duress.

Ransom demands

Security services advise the families of those kidnapped not to speak to the media, as they believe comments made could undermine behind-the-scenes efforts to negotiate the release of those held.

The US and UK have accused European countries to ceding to the militants demands for cash.

A New York Times investigation found that Isis had netted $125 million from ransoms, many allegedly paid by France.

In April, four French journalists were freed by Isis. One, Didier Francoise, has spoken of being held with US journalist James Foley, one of the four hostages executed by Isis.

Two Spanish journalists held hostage were released in March.

It is not just western hostages that have been murdered on camera by the jihadist group.

In August a Kurdish peshmerga fighter, seized battling Isis militants in Iraq was executed on camera, wearing the same orange jumpsuit as western captives.

Experts believe that the videos seek to spread terror amongst those battling them, demonstrating the group's implacability and ruthlessness.