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ISIS militants on Friday released a video showing the beheading of British aid worker Alan Henning, in another gruesome execution that U.K. Prime Minister David Cameron condemned as “barbaric and repulsive” in vowing to bring those responsible to justice.

Henning’s execution is the fourth time the Islamist militants beheaded a Western hostage and released a video intended to send a message to American and British governments who have undertaken a campaign to combat the group. An American hostage is also threatened with execution.

In the new video, titled "Another Message to America and Its Allies," a black-clad, masked militant with what appears to be a British accent forces the captive to read a brief statement before the execution, as in the other videos. The hostage is forced to say he “will pay the price” for Britain’s decision to join a U.S.-led bombing campaign against ISIS in Iraq.

At the end of the video, American hostage Abdul-Rahman Kassig is shown and threatened with execution next.

Kassig, 26, a native of Indiana, is an Army veteran who served in Iraq, his family said. He was captured in Syria on Oct. 1, 2013, while working in that country as part of a relief organization he founded, Special Emergency Response and Assistance, his family said in a statement. He changed his name from Peter while in captivity.

Cameron condemned the murder and vowed to bring those responsible to justice. “The brutal murder of Alan Henning by ISIL shows just how barbaric and repulsive these terrorists are,” he said.

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

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

The U.K. launched its first airstrikes against ISIS in Iraq on Tuesday, four days after Parliament voted to authorize military force against the terror group. A U.S.-led coalition has conducted nearly 300 airstrikes against the Islamist militant group in Iraq and Syria since August.

President Barack Obama said the U.S.-led campaign against ISIS would continue.

“The United States strongly condemns the brutal murder of United Kingdom citizen Alan Henning by the terrorist group ISIL. 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 in a statement.

“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.”

ISIL is another name used by the terror group that has seized large parts of Iraq and Syria.

Henning was driving an ambulance in an aid convoy when he was taken hostage in December. ISIS first publicly threatened his life in September at the end of a propaganda video showing the beheading of another British citizen, David Haines. In that video, a black-clad militant gripping a knife warned Henning would be next to die if the U.K. continued to aid U.S. efforts against ISIS.

Henning’s wife has pleaded with ISIS for mercy in a statements released over recent weeks, saying Henning was a “peaceful, selfless man” who gave up his job as a taxi driver to help Syrians in need.

“I cannot see how it could assist any State's cause to allow the world to see a man like Alan dying,” Henning’s wife Barbara said in the statement.

Haines’ family had made a similar public appeal to ISIS just hours before the video of his execution emerged. ISIS militants have murdered two American journalists — James Foley and Steven Sotloff. A third British hostage, photographer John Cantlie, has appeared in a vastly different propaganda effort: a video promising a series of “programs” that will illuminate “the truth” about ISIS.

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The brutal murder of Alan Henning by ISIL shows just how barbaric these terrorists are. My thoughts are with his wife and their children. — David Cameron (@David_Cameron) October 3, 2014

— Cassandra Vinograd and Phil Helsel