(Transcript from SBS World News Radio)
United States president Barack Obama has described the brutal beheading of a US aid worker, Peter Kassig, as an act of pure evil.
The self-proclaimed Islamic State, or IS, has claimed responsibility for the murder, releasing a video showing the aftermath of the killing.
It comes as the top-ranked US general, General Martin Dempsey, says the battle with IS is starting to turn.
Biwa Kwan reports.
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26 year old Peter Kassig was captured last year during a mission to deliver hospital supplies to Syria.
The United States aid worker had returned to the Middle East to treat the wounded after serving as an army ranger in Iraq in 2007.
In 2012, he told CNN the decision was motivated by a need to help those in need.
"We each get one life, and that's it. You get one shot at this. We don't get any do-overs, you know? And, for me, it was kind of, 'Put up or shut up.' The way I saw it, I didn't have a choice. This is what I was put here to do."
Now, he has become the fifth Western hostage to be killed by IS.
World leaders and Mr Kassig's family have condemned the ensuing boasting by the group in a video as appalling and horrifying.
The White House has authenticated the video, which shows a masked man standing over Mr Kassig's severed head.
It also shows the beheading of 18 Syrian captives.
The militant says Mr Kassig's death is a warning to the West.
US president Barack Obama says the act is "pure evil" carried out by a terrorist group.
In a statement, the aid worker's parents, Ed and Paula Kassig, say they are heartbroken at the death of their son but proud of his selfless work in Syria.
Friend and journalist Michael Downey says it is significant that Peter Kassig did not deliver a propaganda statement in the video.
He says the courage that refusal would have taken fits with what friends and family knew of the man's character.
He told the BBC Mr Kassig will be remembered for his positive impact on the lives of others.
"He was an incredible person -- unrelenting positivity. After covering the region for a while, it's very easy to be a bit jaded and negative, and he'd call you out and say, 'Well, okay, what's the point in focusing on the negativity or dwelling on that?' He truly believed that if he could help one person -- you know, even if it's just one person -- that still makes a difference."
The IS militant who beheaded him is believed to be the man known as "Jihadi John," the same man who claimed the murder of British aid worker Alan Henning.
British prime minister David Cameron says he is horrified by the killing.
He says efforts to hold so-called "Jihadi John" to account are continuing.
"We should be in no doubt that I want Jihadi John to face justice, you know, for the appalling things, the appalling acts, that have been carried out in Syria."
Separately, it is believed IS is behind two new bombings near Baghdad International Airport.
One of the explosions targeted a United Nations convoy heading into Baghdad's heavily fortified Green Zone.
A vehicle was damaged, but no one was seriously hurt.
The other attack was a suicide bombing, and IS has claimed responsibility.
Analysts says the timing of the video's release is no coincidence.
A Middle East expert at the London School of Economics and Political Science, Fawaz Gerges, has told CNN the murder of Mr Kassig shows IS is growing desperate.
"This is the only means for the so-called Islamic State to retaliate against the Americans and the Western powers. They have no other means to do so. They are being squeezed. They are on the defensive. Their leadership is being targeted by the Americans. They're hurting. Their advance has been basically stopped. They have lost hundreds of, if not more than a thousand, skilled fighters in the last few weeks."
During an unannounced visit to Iraq at the weekend, the leading US general, General Martin Dempsey, said the US-led coalition campaign is already having an effect in Iraq.
He says the US military advisers, along with the Iraqi government, are already building a fighting force with Sunni Muslim groups in the region.
But he warns the campaign is likely to last several years.