Alan Henning beheading by ISIS purportedly shown in internet video

An online video released Friday purports to show an Islamic State group fighter beheading British hostage Alan Henning, a former taxi driver who was taken captive while travelling with an aid convoy.

The video mirrored other beheading videos shot by the Islamic State group, which now holds territory along the border of Syria and Iraq.

"The brutal murder of AlanHenningbyISILshows just how barbaric these terrorists are. My thoughts are with his wife and their children," British Prime Minister David Cameron said Friday on Twitter.

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

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Before he left the U.K.. to dohumanitarian work, the father of two drove a taxi in the communitynot far outside Manchester. His wife, Barbara, had pleaded for his release, calling her husband a "peaceful, selfless man" who wanted to help those in need.

"Alan had gone to Syria to help get aid to people of all faiths in their hour of need," Cameron said in a statement."Thefact 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,- British Prime Minister David Cameron

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

The Associated Press could not immediately verify the video's authenticity, though it was released in the same manner as other Islamic State group videos.

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 U.S. military efforts against the Islamic State group by using its forces to help with logistics and intelligence gathering, as well as by recently taking part in airstrikes in Iraq. The internet video released Friday begins with a news clip announcing British strikes against ISIS.

Threat against American

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

Two U.S. 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 Islamic State militants. Theofficials declined to elaborate.

The family of the former Army Ranger from Indianasays the man moved to the Middle East to provide humanitarian aid to refugees.Kassig's parents say their son converted to Islam while in captivity and goes by the name Abdul-Rahman. They say he served in the Iraq war and became an emergency medical technician after being honourably discharged for medical reasons.

Kassig travelled to Lebanon in 2012 to work as a medical assistant in border hospitals. He has been held since October 2013.

"Obama, you have started your aerial bombard of Shams [Syria], which keep on striking our people, so it is only right that we strike the next of your people," a masked militant said in the video.

U.S. President Barack Obama condemned the "brutal murder" of Henning, and said the U.S. will work with U.K. allies "to bring the perpetrators of Alan's murder as well as the murders of Jim Foley, Steven Sotloff and David Haines to justice."

This is the fourth such video released by the Islamic State group. The fullbeheadingsare not shown in the videos, but the British-accented, English-speaking militant holds a long knife and appears to begin cutting the three men.

Foley and Sotloff were reportersand Haines was an aid worker.

The video was released the same day that Prime Minister Stephen Harper said Canada has offered to contribute an air combat mission to the fightagainst ISIS, in addition to the deployment of military advisers who are assisting in Iraq.