BEIRUT (AFP, Reuters) - The Islamic State in Iraq and Syria (ISIS) on Sunday claimed to have executed US aid worker Peter Kassig and warned the United States they would kill other US citizens "on your streets".
The 15-minute video also showed the gruesome simultaneous beheadings of at least 18 men described as Syrian military personnel, the latest in a series of mass executions and other atrocities carried out by ISIS.
"This is Peter Edward Kassig, a US citizen of your country," said a black-militant wearing a balaclava, the same outfit worn by the man who beheaded two American journalists and two British aid workers in earlier videos.
The man stood over a severed head bearing a resemblance to Kassig, a former American soldier who risked his life to provide medical treatment and aid to those suffering from Syria's civil war.
"Here, we are burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive," the militant said in a British accent. Dabiq is the site of a major 16th century battle in what is now northern Syria that saw the Ottomans defeat the Mamluks and begin a major expansionist phase of an empire the ISIS group considers to have been the last caliphate.
In a highly choreographed sequence earlier in the video, ISIS militants marched at least 18 prisoners said to be Syrian pilots and officers loyal to Syrian President Bashar al-Assad by a wooden box of long military knives, each taking one as they passed, then forced them to kneel in a line and decapitated them.
In Washington, President Barack Obama's National Security Council (NSC) said the US government was working to confirm the authenticity of the claim. "If confirmed, we are appalled by the brutal murder of an innocent American aid worker and we express our deepest condolences to his family and friends," NSC spokeswoman Bernadette Meehan said in a statement.
Other Western leaders and officials also condemned the killing.
ISIS spearheaded a militant offensive that overran much of Iraq's Sunni Arab heartland since June after seizing major territory in neighbouring Syria, and carried out a series of atrocities in both countries. The group has killed hundreds of Iraqi and Syrian tribesmen who opposed it, attacked religious and ethnic minorities, sold women as slaves, executed scores of Iraqi security personnel and carried out beheadings on camera.
Sunday's video was released as ISIS suffered battleground losses in Iraq backed by US-led air strikes.
Kassig, a 26-year-old from Indiana, is also known as Abdul-Rahman, a name he took after his conversion to Islam while in captivity. His parents have said through a spokesperson their son was taken captive on his way to the eastern Syrian city of Deir al-Zor on Oct. 1, 2013.
A former soldier, Kassig was doing humanitarian work through Special Emergency Response and Assistance, an organisation he founded in 2012 to help refugees from Syria, the family has said.
Islamic State has previously killed US journalists James Foley and Steven Sotloff and British aid workers David Haines and Alan Henning.