LONDON - Prime Minister David Cameron vowed Sunday that Britain would do all it could to catch the killers of a British aid worker beheaded by the Islamic State, which he condemned as an "act of pure evil".

In its third beheading of a Western hostage in less than a month, IS released a video late Saturday showing the execution of Briton David Haines and threatening the life of another British captive.

As President Barack Obama offered US support for its "ally in grief", Cameron faced growing calls at home to commit Britain's military to Washington's planned assault against the jihadist group that has seized parts of Syria and Iraq.

US Secretary of State John Kerry was in Paris to push for a broad international coalition against IS, which has already secured the backing of 10 Arab states including Saudi Arabia.

The bid was boosted on Sunday by Australia's announcement that it was deploying 600 troops to the region to join the effort against what Prime Minister Tony Abbott called a "murderous death cult".

France is hosting an international conference on Iraq on Monday, and President Francois Hollande's office said the "heinous killing" of Haines was another reason why a global push against IS was needed.

Cameron chaired a meeting of the government's emergency Cobra committee early Sunday in response to the online video, which he said showed "a despicable and appalling murder of an innocent aid worker" and "an act of pure evil."

The footage shows a masked IS militant claiming the execution was in retribution for Britain's role in the campaign against the group.

Britain has not joined US air strikes against IS in Iraq, but has begun arming Kurdish peshmerga fighters battling the militants in the north of the country.

"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," Cameron said.

Obama slammed the latest killing as "barbaric" and said the US "stands shoulder to shoulder tonight with our close friend and ally in grief and resolve".

Two US journalists were murdered in similar circumstances in recent weeks in two videos posted online, and Haines was threatened in the last one showing the beheading of reporter Steven Sotloff.

The Foreign Office in London said the two and a half minute video released this weekend, entitled "A Message to the Allies of America", appeared genuine.

It opens with a clip of Cameron outlining how Britain was 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, the militant threatens to execute another British captive, Alan Henning.

In a moving statement, Haines's brother Mike paid tribute to a "good brother" who he said was "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," he said, adding that he would be "missed terribly".

Haines, 44, was taken hostage in Syria in March 2013 while working for the French NGO Acted, which condemned his death as a "barbaric crime" that "cannot go unpunished".

The former head of the British army, Richard Dannatt, piled pressure on Cameron to take action.

"If we do not confront and destroy these Islamic State jihadi fighters then their influence will grow, their confidence will grow and the problem will get bigger," he told Sky News television.

Obama on Wednesday set out a strategy that would include air strikes in Syria and expanded operations in Iraq, where US aircraft have carried out more than 160 strikes since early August.

Announcing that Australia would deploy 600 troops to the United Arab Emirates as part of the US-led effort, Abbott said further decisions were needed before he would commit to combat operations in Iraq.