A video has been released on the internet that purports to show an ISIL fighter beheading British hostage Alan Henning, the fourth such killing carried out by the armed group now being targeted by US-led air strikes.

The video, released on Friday, begins with a news clip announcing British strikes against ISIL, or the Islamic State of Iraq and the Levant.

The authenticity of the video could not immediately be verified, though it was released in the same manner as other ISIL videos and the masked man sounded similar to the one who carried out the other slayings.

Henning, 47, nicknamed "Gadget", had joined an aid convoy and was taken captive on December 26, shortly after crossing the border between Turkey and Syria.

Earlier this week, Henning's wife Barbara Henning asked ISIL in a televised plea: "Please release him. We need him back home."

In a statement, the British Foreign Office said it was working to verify the video.

"If true, this is a further disgusting murder," the statement read. "We are offering the family every support possible; they ask to be left alone at this time."

Britain has been supporting US military efforts against ISIL by using British forces to help with logistics and intelligence gathering, as well as recently taking part in air strikes in Iraq.

'British hero'

Dozens of Muslim leaders in Britain had urged ISIL to release Henning. His wife had said she had been given hope by "the outcry across the world" over her husband's imprisonment.

Mohammed Shafiq, chief executive of the Ramadhan Foundation, a Muslim charity, called Henning "a British hero".

His "barbaric killing is an attack against all decent people around the world", Shafiq said.

David Cameron, UK prime minister, said Henning's apparent slaying showed "how barbaric and repulsive these terrorists are".

"Alan had gone to Syria to help get aid to people of all faiths in their hour of need," Cameron said in a statement.

"The fact that he was taken hostage when trying to help others and now murdered demonstrates that there are no limits to the depravity of these ... terrorists.

"We will do all we can to hunt down these murderers and bring them to justice."

US President Barack Obama's counterterrorism adviser, Lisa Monaco, said the US had seen the video and was evaluating it.

"This is again yet another just very clear example of the brutality of this group, and why the president has articulated and is moving out in a comprehensive way to degrade and destroy ISIL," Monaco said.

"Our hearts go out to the British aid worker who we believe is in that video, and to the remaining hostages and to their families."

The full beheadings are not shown in the ISIL videos, but the British-accented, English-speaking masked man holds a long knife and appears to begin cutting his victims, which include American reporter James Foley, American-Israeli journalist Steven Sotloff, British aid worker David Haines and now Henning.

American hostage

The video ended with a fighter threatening a man identified as an American named Peter Kassig.

"Obama, you have started your aerial bombardment of Sham [Syria], which keeps on striking our people, so it is only right that we continue to strike the neck of your people," the masked man in the video said.

James Comey, FBI director, has said American officials believe they know the identity of the masked man, though he has declined to name the man or reveal his nationality.

Two US officials, who spoke on condition of anonymity out of concerns of not having permission to release the information, confirmed that Kassig was being held by ISIL fighters. The officials declined to elaborate.

Kassig, a 26 year-old American, enlisted in the army in 2004, and became a Ranger, ultimately serving in the 75th Ranger Regiment, an army special operations unit.

According to his military record, Kassig trained at Fort Benning, Georgia in 2006, and deployed to Iraq from April to July 2007.

He was medically discharged at the rank of private first class in September 2007.