The militant group that calls itself the Islamic State has released a video that shows the beheading of British citizen Alan Henning.

The video is made in much the same style as the others, and shows a masked militant begin to cut the head off a man who looks like Henning.

"The brutal murder of Alan Henning by ISIL shows just how barbaric and repulsive these terrorists are," Prime Minister David Cameron said, using an alternate name for the militant group.

Henning, who was assisting an Islamic charity with an aid convoy in Syria when he was kidnapped, is the fourth Westerner the Islamic State has killed.

At the end of the video, the militants show another hostage, who they claim is an American citizen.

In the video, Henning says he is paying the price for his country's decision to join a coalition fighting a war against the Islamic State.

Earlier this week, Henning's wife, Barbara, issued a direct appeal to the Islamic State to free her 47-year-old taxi driver husband.

"I ask Islamic State, please release him, we need him back home," she said.

Barbara said Henning was innocent, that he was in Syria volunteering with his Muslim friends to help the people of Syria.

The Islamic State previously has released videos showing the beheading of American journalist Steven Sotloff and James Foley and British aid worker David Haines.

Update at 7:10 p.m. ET. American Hostage Is Former Soldier:

The man shown at the end of the video released by the Islamic State is 26-year-old Abdul-Rahman Kassig, according to a statement by released by the man's family.

"We ask everyone around the world to pray for the Henning family, for our son, and for the release of all innocent people being held hostage in the Middle East and around the globe," the family said.

Kassig was in Syria doing humanitarian work.

Update at 6:19 p.m. ET. A Brutal Murder:

In a statement, President Obama condemned the "brutal murder" of Henning.

"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. "Standing together with our UK 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. Standing together with a broad coalition of allies and partners, we will continue taking decisive action to degrade and ultimately destroy ISIL."

Update at 5:13 p.m. ET. 'Barbaric And Repulsive':

"The brutal murder of Alan Henning by ISIL shows just how barbaric and repulsive these terrorists are," Prime Minister David Cameron said in a statement. "My thoughts and prayers tonight are with Alan's wife Barbara, their children and all those who loved him."

He continues:

"Alan had gone to Syria to help get aid to people of all faiths in their hour of need. 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 ISIL terrorists.

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

Update at 5:03 p.m. ET. Evaluating Authenticity:

Lisa Monaco, assistant to president on Homeland Security, said during a scheduled press conference that the Obama administration was aware of the video and was evaluating its authenticity.

If it turns out to be authentic, she said, it's "another demonstration of the brutality of" Islamic State and why President Obama has moved to "degrade and ultimately destroy" the group.

Britain's Foreign & Commonwealth Office issued a similar statement.

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

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