A video released on the Internet on Friday appears to show British hostage Alan Henning being beheaded by a masked Islamic State fighter, the militant-monitoring SITE Intelligence Group reported.
Henning, 47, a former taxi driver from the Manchester area, had been held captive in Syria for nine months after traveling to the crisis-torn region to assist with humanitarian relief work in the more than 3-year-old civil war.
Islamic State fighters have seized broad swaths of Syria and Iraq and in June proclaimed a "caliphate" in that territory, where they have rampaged against fellow Muslims and adherents of other faiths. Thousands have been killed by the group, provoking U.S.-led airstrikes aimed at its strongholds.
The 71-second video released Friday, titled "Another Message to America and Its Allies," also shows the militant threatening another hostage, whom SITE identified as Peter Edward Kassig, an American aid worker.
"Obama, you have started your aerial bombardment of Shams [Syria], which keeps on striking our people, so it is only right that we strike the necks of your people," the dagger-wielding man says, according to a translation of the video by the Associated Press in Cairo.
The AP said it couldn't verify the video's authenticity but described it as similar in content and format to three other recordings released by Islamic State depicting the executions of American journalists James Foley and Steven Sotloff and Scottish aid worker David Haines.
Islamic State also claimed to have orchestrated the execution of a fifth Western captive, French tourist Herve Gourdel, who was seized while trekking in Algeria's mountainous Kabylie region Sept. 21 and shown being decapitated in a video obtained by SITE Intelligence Group two days later. Gourdel, 55, was killed by an Islamic State-affiliated militant group known as Jund Khilafah, or Soldiers of the Caliphate.
Kassig, whose detention by Islamic State hadn't been widely disclosed before Friday, has been identified as an Indiana native and former U.S. Army Ranger. A January 2013 Time magazine article on Kassig, 26, said he had formed an aid group after his discharge from the Army to help Syrian refugees in Lebanon.
National Security Council spokeswoman Caitlin Hayden confirmed late Friday that Kassig was known to be held by Islamic State and said the United States "will continue to use every tool at our disposal" to secure his safe release.
A statement issued on behalf of Kassig's parents, Ed and Paula Kassig of Indianapolis, said the family "extends our concern for the family of Alan Henning. We have read about his work and his generous character with great respect and admiration. 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."
A statement issued by the White House condemned Henning's "brutal murder."
"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. 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," the Obama administration said. "Standing together with a broad coalition of allies and partners, we will continue taking decisive action to degrade and ultimately destroy ISIL."
ISIL is one of several acronyms for Islamic State.
The video released Friday appears to show the same British-accented executioner seen in the Foley, Sotloff and Haines slayings.
Henning, a married father of two known to his friends as "Gadget" for his technical savvy, was kidnapped in Ad Dana, Syria, shortly after crossing the border with an aid convoy from Turkey in late December.
British authorities said they were still examining the video and had no immediate comment.
A second video purportedly from Islamic State that was posted on YouTube Friday — and later removed by the sharing platform — was titled "Message of the Mujahid," Arabic for "religious fighter." It showed an unmasked militant in camouflage clothing calling on Muslims in Britain to rise up and attack to "cause terror in the hearts" of the British people.
Speaking with a British accent like the shrouded figure who appeared in the beheading videos, the bearded, bespectacled militant called British Prime Minister David Cameron "a despicable swine" for backing U.S. airstrikes and taunted him to send British ground forces against Islamic State so that the militants can "send them back one-by-one in coffins."
The earlier threats against Henning prompted British Muslim organizations and key figures within Britain's Muslim community to appeal for his release.
Henning's wife, Barbara, had issued several messages directed at Islamic State asking that her husband be freed and highlighting his desire to help the people of Syria.
"When he was taken, he was driving an ambulance full of food and water to be handed out to anyone in need," she said. "His purpose for being there was no more and no less. This was an act of sheer compassion."
The family later received an audio recording of Henning pleading that his life be spared.
Amandla Thomas-Johnson, a spokesman for Cage, a British organization that had campaigned for Henning's release, described him as "courageous and brave" and "willing to put others' suffering ... before the relative luxuries of life in the UK."
He denounced Islamic State's behavior as inconsistent with Islamic law," but also criticized the British government for not doing more to secure Henning's release, and for its decision to join U.S.-led airstrikes against the militants.
Special correspondent Werth reported from London and Times staff writer Williams from Los Angeles.