It is feared Islamic State holds at least 24 western hostages, including Britons.

ISIS may have 24 western hostages including Britons and even jihadists who changed their minds

The number of hostages is also estimated to include a few British jihadists who wanted to return home after being reluctant to commit murder.

Mostly aid workers and journalists, their identities are often kept secret by their home nations in case details about them become bargaining chips for their captors.

France, Spain and Italy have reportedly paid ransoms to free hostages and fighters have pocketed an estimated £45million.

When aid worker David Haines was taken at the Atmeh refugee camp in Syria’s Idlib province last year, he was snatched with co-worker Federico Motka from Italy. Motka was released last May.

In August, Italian aid workers Greta Ramelli, 20, and Vanessa Marzullo, 21, were grabbed in Aleppo.

Two Spanish and four French journalists have been freed this year. IS is also holding a 26-year-old American woman who was doing humanitarian work.

She is the third American known to have been taken by them.

US journalist Austin Tice vanished in Syria in August 2012.

Last November, the New York-based Committee to Protect Journalists said at least 30 have been kidnapped or disappeared in Syria.

The group reported 52 journalists have been killed since Syria’s civil war began in 2011 and records at least 24 who vanished earlier this year but are now safe.

Elsewhere, Paris-based Reporters Without Borders claims at least 60 “news providers” are being detained and more than 110 have been killed.

It is also believed that Western hostages include UK nationals who went to fight for IS but then decided to try to return home has been held captive.

An audio-visual forensic expert said, the videos of David Haines, James Foley and Steven Sotloff are unlikely to show their actual murders, but have instead been released as “propaganda”, it has been claimed.

Lack of blood and screams in the films suggested the terror acts had not been carried out at that time.

Analysing the footage of David, he also said the British aid worker’s body did not “slump, buckle or draw away from the knife and no body resistance can be observed”.

He added: “I would conclude this scene is more for propaganda purposes. I am of the opinion this is not a video of murder taking place.”

He said terrain and sky colour suggested the footage of David and James was shot on different days.