Islamic State (ISIS) has beheaded US citizen and aid worker Peter Kassig, according to video footage posted on social media - even though the hostage converted to Islam. 

Kassig, who had converted and changed his name to Abdul-Rahman Kassig, is the fifth Western hostage to be murdered in ISIS's brutal social media campaign against the West. 

Kassig, who served during the Iraq war before being medically discharged, became an emergency medical technician; he travelled to Lebanon in May 2012, volunteering in hospitals and treating Palestinian refugees and those fleeing Syria’s nearly four-year civil war.

His family released a plea to ISIS in October, begging the group to spare his life.

But he was beheaded regardless, the footage purportedly reveals - along with 'dozens' of other men apparently murdered in the video, which is over 15 minutes long. 

AFP reports Sunday that the video shows a long line of victims, including Kassig, being forced to take the knives that kill them from a wooden box as they proceed in a funeral march. According to the report, it is "a highly choreographed sequence."

ISIS has beheaded hostages following strategic developments in the international airstrike campaign against it in Iraq and Syria, including the first official announcement of the campaign and after different countries, e.g. the UK, joined the strikes. 

This time, Kassig's murder is revealed days after US General Martin Dempsey stated to international media that significant progress was being made in the fight against ISIS, and that while it could take years, ISIS has begun to suffer strategic losses. 

It also surfaces hours after news broke that the infamous 'Jihadi John,' the alleged executioner behind all the videos, including this one, may have been injured or killed in airstrikes last week.