ISIS Claims to Have Killed Another Hostage

British aid worker Alan Henning had been held hostage in Syria since December 2013.

The Islamic State of Iraq and Syria released a video Friday afternoon purportedly showing the beheading of British aid worker Alan Henning, the Associated Press reports.

Henning, a 47-year-old cab driver from Manchester, had been held hostage in Syria since last December. He had been in the country as part of an aid convoy to bring medical supplies to refugees.

In the video, a disguised member of ISIS also threatened the life of Peter Kassig, an American, according to BuzzFeed. Kassig, a former U.S. soldier, had been treating Syrian refugees in a Lebanese hospital, but it's unclear whether he had been abducted. ISIS claims to have at least two other hostages—an American aid worker and a British freelance journalist. U.S. or British officials have not yet verified the authenticity of this latest video.

Henning, a father of two, would be the fourth Western hostage to be killed by ISIS militants this year. In August, the extremist group released a video showing the murder of American journalist James Foley. At the time, the White House called Foley's murder "a terrorist attack." On Sept. 2, ISIS released footage of the murder of Steven Sotloff, another American journalist. On Sept. 13, a video showed the execution of David Cawthorne Haines, also a British aid worker. Henning was shown at the end of that video.

Earlier this week, Henning's wife, Barbara, made a televised plea to ISIS to set her husband free.

"We are at a loss why those leading Islamic State cannot open their hearts and minds to the truth about Alan's humanitarian motives for going to Syria," she said. "Please release him. We need him back home."

She said she'd received an audio message last week from her husband pleading for his life, and that she'd been told Henning, after being tried in a sharia court, was found innocent of being a spy.

President Obama announced last month a plan to "degrade and ultimately destroy" ISIS. Friday's video is the first footage released after a U.S.-led coalition launched airstrikes on the terrorist group in Syria and Congress passed legislation to arm moderate Syrian rebels in the fight.

Last week, British Prime Minister David Cameron devoted his United Nations General Assembly address to the need to combat ISIS. "We should be uncompromising—using all the means at our disposal—including military force to hunt down these extremists," he said.

On Friday, Cameron sent his condolences to Henning's family.

"The brutal murder of Alan Henning by ISIL shows just how barbaric these terrosists are," he tweeted. "My thoughts are with his wife and their children."

An official statement from the British government on Friday night added to Cameron's tweet.

"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 [ISIS] terrorists," the statement read. "We will do all we can to hunt down these murderers and bring them to justice."

Obama condemned the slaying of Henning in a statement Friday night. "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," he said.

Marina Koren contributed to this article