DAVID Cameron has condemned the "sickening murder" of a Jordanian pilot burned to death by Islamic State (IS) extremists.
The Prime Minister said the barbaric act would "only strengthen" the resolve of the international community to defeat the militants.
A video posted online appears to show Lt Moaz al Kasasbeh standing in a cage before being set on fire.
The pilot was captured by IS when his plane came down near their Raqqa stronghold in Syria in December while on a mission supporting US-led airstrikes.
Jordanian state TV confirmed the death, suggesting he was actually killed a month ago.
Posting on Twitter, Mr Cameron said: "Lieutenant Moaz al Kasasbeh's sickening murder will only strengthen our resolve to defeat ISIL (IS). My prayers are with his family tonight."
The pilot's fate has gripped Jordan and provoked rare protests against King Abdullah over the government's handling of the case.
US President Barack Obama said the video, if real, would redouble the determination of a U.S.-led alliance to degrade and destroy Islamic State. He said it would be another sign of the "viciousness and barbarity" of the militants.
Michael Haines, brother of the murdered British IS hostage David Haines, said: "My heart is filled with sadness yet again to hear of the savage murder of another hostage by the terrorists (IS).
"Firstly, my thoughts and prayers go out to the family of Moaz al Kasasbeh, I know only too well the feelings of loss, anger and disbelief they must be experiencing right now. I share their pain.
"The murder of Moaz al Kasasbeh once more shows the unmerciful nature of these criminals.
"The world must come and stand together in the rejection of the fear the terrorists are attempting to spread. Only when we are all united against this vile threat will we defeat it."
Former Liberal Democrat leader Sir Menzies Campbell said: "These events simply underline that there is no level of barbarity to which IS is not willing to sink in support of its cause."
In the video, the burned man wore orange clothes similar to those worn by other foreign Islamic State captives who have been killed since a US-led coalition started bombing the militants in July.
Islamic State has released videos showing the beheadings of several Western hostages and said that it has killed two Japanese captives.
The militants have come under increased military pressure from air strikes and a push by Kurdish and Iraqi troops to reverse their territorial gains in Iraq and Syria.
In the video, Lt al-Kasaesbeh is interviewed, describing the mission he was due to carry out before his jet crashed. The video also showed footage of the aftermath of air strikes, with people trying to remove civilians from debris.
A man resembling Lt al-Kasaesbeh is shown inside the cage with his clothes dampened, apparently with flammable liquid, and one of the masked fighters holds a torch, setting alight a line of fuel which leads into the cage.
The man is set ablaze and kneels to the ground.
Fighters then pour debris, including broken masonry, over the cage which a bulldozer then flattens, with the body still inside. The video showed a desert setting similar to previous videos of killings.
Jordan had been demanding the release of Lt al-Kasaesbeh in exchange for an Iraqi woman Islamic State was seeking to swap for Japanese hostage Kenji Goto. The beheading of Goto, a veteran war reporter, was shown in a video released by Islamic State on Saturday.
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