ISIL fighters released a graphic video Sunday purporting to show the beheading of captured U.S. aid worker Peter Kassig.
The clip posted online did not show the murder itself, but showed a masked man standing with a decapitated head covered in blood lying at his feet. Speaking in English in a British accent, the man says: "This is Peter Edward Kassig, a U.S. citizen."
The White House later said that based on a reivew of the video, it believed the murder of Kassig was genuine. President Barack Obama responded to the latest beheading of a U.S. citizen by the extremist group by condeming the killing as an "act of pure evil."
Kassig, a 26-year-old from Indiana, is also known as Abdul-Rahman, a name he took following his conversion to Islam while in captivity.
A former soldier, Kassig was doing humanitarian work through Special Emergency Response and Assistance, an organization he founded in 2012 to help refugees from Syria, the family has said.
If confirmed, Kassig's beheading would be the fifth such killing of a Westerner by the Islamic State of Iraq and the Levant (ISIL), following the deaths of two U.S. journalists and two British aid workers.
In an apparent reference to testimony about Kassig by former fellow captives, the masked man says: "Peter, who fought against the Muslims in Iraq while serving as a soldier under the American army, doesn’t have much to say. His previous cellmates have already spoken on his behalf."
The announcement of Kassig's death formed part of a 15-minute video in which the group issued warnings to the United States, Britain and others including Shi'ite Muslims.
The video identifies the group’s location as Dabiq, a small town in the northern Syrian province of Aleppo, near the Turkish border.
The footage also shows what appears to be the mass beheading of several Syrian soldiers captured by the group. ISIL warn that U.S. soldiers will meet a similar fate.
"We say to you, Obama...you claim to have withdrawn from Iraq four years ago," the masked extremist said. "Here you are: you have not withdrawn. Rather, you hid some of your forces behind your proxies," he said, apparently referring to Western-backed Syrian rebels, Kurdish fighters and the Iraqi military.
"Here we are, burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive."
Kassig, a former U.S. Army Ranger, was providing medical aid to Syrians fleeing the civil war when he was captured inside Syria on Oct. 1, 2013.
The latest video did not show the person identified as Kassig being beheaded. Unlike previous videos, it did not show other Western captives or directly threaten to behead anyone else.
The group also holds British photojournalist John Cantlie, who has been shown in several videos delivering long statements in English on the group's behalf, perhaps under duress.
British Prime Minister David Cameron said he was horrified by the latest "cold-blooded murder". Britain's Foreign Office said it was analyzing the video.
Kassig's family said in a statement that they "respectfully asks that the news media avoid playing into the hostage takers' hands and refrain from publishing or broadcasting photographs or video distributed by the hostage takers."
"We prefer our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause," the statement added.
U.S. National Security Council spokeswoman Bernadette Meehan said that if the video is authentic, the White House would be "appalled by the brutal murder of an innocent American."
ISIL has beheaded and shot dead hundreds of captives — mainly Syrian and Iraqi soldiers — during its brutal sweep across the two countries, and has celebrated its mass killings in a series of slickly produced, extremely graphic videos.
The group has declared an Islamic caliphate in the areas under its control in Syria and Iraq, which it governs according to its own extremely violent interpretation of Shariah law.
The U.S. began launching air strikes in Iraq and Syria earlier this year in a bid to halt the group's rapid advance and eventually degrade and destroy it.
The extremist group emerged from the remains of al-Qaida in Iraq and spread to Syria, where it battled both government forces and rebel groups as it carved out its self-styled Islamic state.
In June the group swept into northern Iraq, capturing about a third of the country, including the second largest city Mosul, and eventually prompting the U.S. to resume military operations in the country less than three years after withdrawing. In September the U.S. expanded the air campaign to Syria.
Al Jazeera and wire services