LONDON - The Islamic State claimed the beheading of a British aid worker on Saturday, an act slammed as "pure evil" by Prime Minister David Cameron who vowed Britain would do all it could to catch the killers.
President Barack Obama offered US support for its "ally in grief", while Cameron faced growing calls to allow Britain's military to help in Washington's planned assault against the rampaging jihadist group.
The British premier will chair a meeting of the government's emergency Cobra committee early Sunday in response to the online video purportedly showing a masked IS militant killing hostage David Haines in retribution for the US and British campaign against the group.
Cameron called the attack "a despicable and appalling murder of an innocent aid worker" and "an act of pure evil."
"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," he said in a statement.
Two US journalists have been murdered in similar circumstances in recent weeks.
Obama slammed the latest attack as "barbaric" and said the US "stands shoulder to shoulder tonight with our close friend and ally in grief and resolve".
Britain has yet to join US air strikes against IS in Iraq, but has offered to arm Kurdish Peshmerga fighters battling against militants in the north of the country, a move cited in the latest video as a reason for revenge.
Murdered 'in cold blood'
Britain's Foreign Office said it was "working as quickly as it could" to verify the two-minute-27-second clip, entitled "A Message to the Allies of America".
The video opens with a clip of Cameron describing the British strategy of working with the Iraqi government to help arm Kurdish fighters against "these brutal extremist militants," and to offer aid, diplomacy, and military help to pressure IS.
Haines then appears, dressed in an orange jumpsuit, and identifies himself before calmly explaining that he is paying the price for Cameron's policy.
The attacker - who appears to be the same man as in the previous two beheading videos - tells Britain the alliance with the US will "accelerate your destruction" and will drag the British people into "another bloody and unwinnable war." At the end of the clip, he also threatens to execute another captive, identified in a caption by name as another British citizen.