It was unclear when the footage was taken and whether the video was released in a bid to hose down reports that British-accented Jihadi John was injured in a US air strike while attending a meeting in an Iraqi town last week.

Kassig's parents Ed and Paula Kassig, who were last night waiting for official confirmation of their son's death, said in a statement: "We are aware of the news reports being circulated about our treasured son and are waiting for confirmation from the government as to the authenticity of these reports."

In the latest video, the man stood over a severed head bearing a resemblance to that of Kassig, a former US soldier who risked his life to provide medical treatment and aid to those suffering from Syria's civil war.

Kassig, a 26-year-old from Indiana who was kidnapped in 2013 in Syria's Deir Ezzor province, also worked in hospitals and clinics treating Syrians who fled to neighbouring Turkey and Lebanon. He was also known as Abdul-Rahman after converting to Islam.

He wrote to his parents saying: "If I do die, I figure that at least you and I can seek refuge and comfort in knowing that I went out as a result of trying to alleviate suffering and helping those in need."

The militant in the video made no mention of Kassig's aid work, only referring to his time as a US soldier in Iraq.

"Here, we are burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive," the militant said.

Dabiq is the site of a 16th century battle that saw the Ottomans defeat the Mamluks and begin expanding an empire that Islamic State considers to have been the last caliphate.

In a highly choreographed sequence earlier in the video, jihadis marched at least 18 prisoners, said to be Syrian officers and pilots, past a wooden box of long black military knives, each taking one as they passed, then forced them to kneel in a line and decapitated them.

Islamic State spearheaded an offensive that overran much of Iraq's Sunni Arab heartland since June after seizing major territory in neighbouring Syria, carrying out atrocities in both countries.

Previous executions of Westerners: 

• August 19, 2014: ISIS posts a video of the decapitation of US freelance photojournalist James Foley, who was seized in northern Syria in November 2012.

The video of Foley’s killing, which provokes worldwide revulsion, also shows a second US reporter, Steven Sotloff, being paraded by a black-clad IS militant who warns that he too will be killed if US President Barack Obama does not stop air strikes on ISIS positions in Iraq.

• September 2, 2014: ISIS says it has beheaded Sotloff, in another propaganda video showing the execution.

The freelance reporter was kidnapped August 4, 2013, in Aleppo near Syria’s border with Turkey. His abduction was kept secret under a news blackout.

• September 13, 2014: ISIS claims to have beheaded British aid worker David Haines, as the US launches a vast diplomatic offensive to build an international coalition against the jihadists.

Haines was seized by militants in March 2013, while working for a Paris-based non-governmental organisation.

• September 24, 2014: ISIS-linked Jund al-Khilifa, or “Soldiers of the Caliphate,” claim to have beheaded French tourist Herve Gourdel following his abduction in Algeria, in a video posted online after Paris rejected their demand to halt air strikes in Iraq.