Islamic State militants have beheaded another American hostage, Peter Kassig, issuing a video claiming the killing on Sunday and warning the United States they would kill other U.S. citizens "on your streets."
U.S. President Barack Obama confirmed the death of the aid worker in what he called "an act of pure evil by a terrorist group that the world rightly associates with inhumanity."
The announcement of Kassig's death, the fifth such killing of a Western captive by the group, formed part of a 15-minute video posted online in which Islamic State showed the beheadings of at least 14 men it said were pilots and officers loyal to Syrian President Bashar al-Assad.
Kassig, 26, from Indiana, was also known as Abdul-Rahman, a name he took after converting to Islam in captivity. His family has said he was taken captive on his way to the Syrian city of Deir al-Zor on Oct. 1, 2013.
"We are heartbroken to learn that our son, Abdul-Rahman Peter Kassig, has lost his life as a result of his love for the Syrian people and his desire to ease their suffering," Ed and Paula Kassig said in a statement. "Our heart also goes out to the families of the Syrians who lost their lives, along with our son."
The video did not show the beheading of Kassig, who previously served in the U.S. Army, but showed a masked man standing with a decapitated head covered in blood at his feet. Speaking in English in a British accent, the man says: "This is Peter Edward Kassig, a U.S. citizen."
The video appeared on a jihadist website and on Twitter feeds used by Islamic State.
In a statement to reporters on Air Force One on his way home from a G20 summit in Brisbane, Australia, Obama praised Kassig's humanitarian work and offered condolences to his family.
The man in the video spoke with the same southern British accent as the killer of previous hostages, dubbed "Jihadi John" by British media. He was believed to have been wounded in an air attack on an IS leaders' meeting in Iraq near the Syrian border earlier this month, some media reports have said.
French daily Le Figaro said on its website that the French Interior Ministry was studying the possibility of the presence of a French national among the Islamic State militants involved in the beheadings shown in the video.
In the video, a masked militant says: "To Obama, the dog of Rome, today we are slaughtering the soldiers of Bashar and tomorrow we will be slaughtering your soldiers," in a prediction that Washington would send more troops to the region to fight Islamic State.
"And with Allah's permission ... the Islamic State will soon ... begin to slaughter your people in your streets."
The format of the video was different from previous such announcements, showing other beheadings in graphic detail, and also showing most of the killers unmasked. The purported location also was disclosed as the northern Syrian town of Dabeq.
An Islamic State supporter in Syria contacted by Reuters said: "The message is very clear. This is what the West understands. They think they can scare us with their planes and their bombs. No, not us. We are out to impose the religion of God and, by his will, we will."
Kassig was doing humanitarian work through Special Emergency Response and Assistance, an organization he founded in 2012 to help Syrian refugees, the family has said.
Obama said Kassig's life stood in stark contrast to the values represented by Islamic State, adding he was a "humanitarian who worked to save the lives of Syrians injured and dispossessed by the Syrian conflict."
Islamic State's "actions represent no faith, least of all the Muslim faith which Abdul-Rahman adopted as his own," Obama added.
Islamic State previously killed U.S. journalists James Foley and Steven Sotloff and British aid workers David Haines and Alan Henning.
The masked militant, who appeared to be the leader of a beheading squad, said Kassig was buried in Dabeq, near the Turkish border.
"Here we are burying the first American crusader in Dabeq. Eagerly waiting for the remainder of your armies to arrive," he says.
Western leaders and officials condemned the killing.
British Prime Minister David Cameron said he was horrified by the "cold-blooded murder."
The beheadings of the Syrian personnel were filmed in death squad style, with militants standing behind a kneeling man in a dark blue overall. In slow motion shots, each of the militants is shown drawing a knife from a box on the side of the road.