The Islamic State's beheading of another Western hostage has strengthened the resolve of a US-led coalition ahead of a Paris conference Monday on how to jointly eliminate the jihadist group.

Prime Minister David Cameron vowed on Sunday that Britain would hunt down the killers of British aid worker David Haines, an act he described as the "embodiment of evil".

Britain was prepared to "take whatever steps are necessary", he said after Haines became the third Western hostage to be beheaded by the militants in less than a month.

IS released a video Saturday showing Haines' killing and a death threat against another British captive, Alan Henning.

President Barack Obama offered US support for its "ally in grief".

A grim-faced Cameron said in a televised statement: "We will hunt down those responsible and bring them to justice, no matter how long it takes.

"Step by step we must drive back, dismantle and ultimately destroy ISIL (IS) and what it stands for. We will do so in a calm, deliberate way but with an iron determination.

"We will not do so on our own, but by working closely with our allies, not just the United States and in Europe, but with our allies in the region."

- 'Murderous death cult' -

Cameron is facing growing pressure at home to take military action against IS, but he made no commitment to joining the United States in launching air strikes on the group in northern Iraq and Syria.

Britain began sending weapons this week to Kurdish fighters battling the militants in northern Iraq, but has faced accusations of confusion over its strategy.

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

"All bases are covered" in the multinational coalition effort, Kerry said in an interview with CBS television aired Sunday, as Washington harnesses diplomatic and public support to smash the militants.

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

The office of French President Francois Hollande, whose country is hosting Monday's international conference on combating IS, said the "heinous killing" of Haines was another reason why a global push was needed.

A European Union foreign policy spokesman called the murder "another demonstration of ISIL's determination to pursue and extend its terror strategy, in breach of all universally recognised values and rights".

Obama slammed the killing as "barbaric" and said Washington stood stands shoulder to shoulder with London "in grief and resolve".

- 'Good brother' -

Haines, 44, who was taken hostage in Syria last year, had previously been shown alive in the video of US journalist Steven Sotloff's killing.

The Foreign Office in London said the latest video, entitled "A Message to the Allies of America", appeared to be genuine.

It opens with a clip of Cameron outlining how London was working with Baghdad to help arm Kurdish fighters against "these brutal extremist militants", and to offer aid, diplomacy, and military help to pressure IS.

Haines then appears, looking gaunt in an orange jumpsuit, and identifies himself, before calmly saying 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 Henning, another captive Briton.

Haines' brother Mike paid tribute to a man who was "most alive" when doing aid work. He had worked in conflict zones in the Balkans, Libya, South Sudan and Syria.

An emotional Haines also read out a passage from the Koran saying: "Since good and evil cannot be equal, repel thou evil with something that is better."

David Haines' Croatian wife Dragana was keeping a low profile at their home in the quiet Croatian town of Sisak.

Obama has 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.

On Sunday, White House chief of staff Denis McDonough told NBC's Meet the Press success will come when IS no longer poses a threat to the United States and its allies in the Middle East.

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