LONDON (AFP) - The Islamic State in Iraq and Syria (ISIS) 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.

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.

Haines's brother Mike paid tribute to a "good brother...who was recently murdered in cold blood." "He was, in the right mood, the life and soul of the party and on other times the most stubborn irritating pain in the ass," he said in a statement.

"He was and is loved by all his family and will be missed terribly." Scottish-born Haines, 44, was taken hostage in Syria in March 2013 and was threatened in a video released this month depicting the beheading by an IS militant of the US journalist Steven Sotloff.

IS released a video claiming the execution of fellow US journalist James Foley on August 19.

Former head of the British Army Richard Dannatt on Sunday piled pressure on Cameron to let the country's military join a planned assault against IS, announced by Obama this week.

"What we absolutely need to do is not be cowed in any way by yet another foul murder of a hostage," he told Sky News.

"We can support them (the US) to confront, attack and defeat the Islamic State jihadi fighters ... and make sure this cancer is removed from the region before it spreads more widely."

Under pressure himself to tackle the problem, Obama on Wednesday set out a strategy which would include air strikes in Syria and expanded operations in Iraq.

But Cameron will be wary of playing into the hands of the captors by escalating tensions and is also recovering from last year's humiliation of failing to achieve parliamentary support for air strikes against Syria's President Bashar al-Assad.

As part of efforts to build up local support for action, US Secretary of State John Kerry on Saturday sought to bolster relations with Egypt during meetings with its leaders.

Egypt's formidable army is unlikely to take part in a military coalition against IS, but the country boasts the prestigious Sunni Muslim authority Al-Azhar, which Kerry said would fight back against the Islamic State's use of the religion.

Kerry takes his push to forge a broad coalition against Islamic State jihadists to France on Sunday , on the eve of an international conference in Paris on peace and security in Iraq.

The CIA put the number of IS fighters at 20,000 to 31,500 in Iraq and Syria, up to three times the previous estimate.

US aircraft have carried out more than 160 strikes in Iraq since early August, the US Central Command said Saturday.