Photos: Obama promises to bring ISIS hostage killers to justice as they threaten to execute Peter Kassig

ISIS have threatened to execute an American war veteran after Jihadi John beheaded British aid worker Alan Henning.

Peter Kassig, 26, a former Army Ranger from Indianapolis, was shown kneeling in the desert at the end of the video of the murder posted online by the terror group.

During the video, the masked militant sends a message to President Obama admitting that U.S. 'aerial bombardments' are putting them under strain, before adding: 'It's only right we continue to strike the necks of your people.'

Mr Henning, a 47-year-old father-of-two, was kidnapped on Boxing Day last year as he delivered aid to Syrian refugees.

The clip, which is in English with Arabic subtitles, shows Jihadi John stepping forward to cut Mr Henning's throat before the screen turns black, then shows his body lying on the desert floor.

Obama strongly condemned his death and urged that US forces and its allies will bring his killers to justice. 

'Mr. Henning worked to help improve the lives of the Syrian people and his death is a great loss for them, for his family and the people of the United Kingdom,' Obama said in a statement.

'Standing together with our U.K. friends and allies, we will work to bring the perpetrators of Alan's murder - as well as the murders of Jim Foley, Steven Sotloff and David Haines - to justice,' Obama said.

The president added that the U.S. and other nations will continue to take action to 'degrade and ultimately destroy' the Islamic State group. 

A spokesman for the National Security Council then said they are doing everything they can to bring home Mr Kassig, who Jihadi John said would be killed next. 

British aid worker Alan Henning has been beheaded by Jihadi John in an ISIS video posted online 

At the end of the footage American war veteran Peter Kassig is shown and threatened with the same fate