The terrorist group Islamic State, also known as ISIL, has apparently killed British aid worker Alan Henning by beheading, six days after the United Kingdom flew its first combat missions in the campaign to stop the ISIL's rampage against hapless Iraqi communities. 

A new video has begun circulating showing the apparent beheading of the British aid worker, who was snatched right after entering Syria with a team of volunteers in December last year. In the same video, ISIL showed another air worker, American Peter Kassig, and threatened his life. It was not clear how Kassig ended up in the hands of the terrorist organization.

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In London, British Prime Minister David Cameron said the "brutal murder" of Alan Henning shows how "barbaric and repulsive" the ISIL could be.

"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." 

Henning was a taxi driver from Manchester who decided to gather up some friends and start a donation campaign to bring food and water to Syrian refugees as the civil war rages there. He was abducted the day after Christmas. 

A U.S. intelligence expert told CNN the new video seemed authentic, although authorities are studying it. If authenticity is confirmed, Henning will be the fourth westerned to be murdered by the ISIL on camera. American journalists James Foley and Steven Sotloff were beheaded this summer, followed by British aid worker David Haines. 

The British foreign office last week released an audio message of Henning pleading for his life. His wife also made a public plea for ISIS to spare Henning's life. Even Muslim leaders joined in the plea, including Shaykh Haitham Al Haddad, a judge on the Shariah Council in London.

"Whatever your grievance with American or British foreign policy, executing this man is not the answer," Al Haddad said.