Elite Indonesian anti-terror police from Densus 88, the country's leading anti-terrorism police squad, escort four Turks arrested on arrival at Jakarta airport on Sunday. The four Turkish nationals were arrested in Sulawesi island and are suspected to be linked to the Islamic State group, a police spokesman said. Photo: AFP

Condemning an act of "pure evil," Prime Minister David Cameron vowed Sunday that Britain would "take whatever steps are necessary" to destroy Islamic State (IS) militants that murdered a British aid worker, but made no commitment to joining the US in airstrikes.

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

After chairing a meeting of the government's emergency Cobra committee, Cameron said the extremists behind the beheading of David Haines were the "embodiment of evil."

"We will hunt down those responsible and bring them to justice, no matter how long it takes," the premier said in a televised statement from Downing Street.

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

Cameron repeated his support for US airstrikes against IS in Iraq, and for President Barack Obama's strategy to build a broad coalition to fight the jihadists.

But despite growing calls at home for action against IS, Cameron made no commitment to British strikes.

During a visit to Berlin this week, UK Foreign Secretary Philip Hammond said Britain would not take part in strikes against IS in Syria, after parliament last year voted against taking military action in that country.

But just hours later, a spokesman for Cameron's Downing Street office insisted the prime minister was not ruling anything out.

Meanwhile, France is to host an international conference on Iraq on Monday and President Francois Hollande's office said the latest killing was further evidence of the need to take action against IS.

The New York Times also reported on Sunday that several Arab countries have offered to carry out airstrikes against IS militants.

Two US journalists were 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."

While not joining in airstrikes currently, Britain has offered to arm Kurdish peshmerga fighters battling the militants in the north of the country, a move cited in the latest video as a reason for revenge.

Britain's Foreign Office said the two-minute-27-second clip, entitled "A Message to the Allies of America," appears to be genuine.

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 British captive, identified as Alan Henning.

Haines, 44, was taken hostage in Syria in March 2013 while working for a French NGO and was threatened in a video released this month showing the beheading by an IS militant of 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.

"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.

Obama on Wednesday set out a strategy that would include airstrikes in Syria and expanded operations in Iraq.

He promised to build a wide-ranging international coalition against the jihadists and on Sunday Australian Prime Minister Tony Abbott said Australia would deploy 600 troops to the United Arab Emirates to join the US-led effort.

"There are obviously further decisions to be taken before Australian forces will be committed to combat operations in Iraq," Abbott told a press conference in Darwin.

"Nevertheless, Australia is prepared to engage in international operations to disrupt and degrade ISIL [IS] because of the threat that this murderous death cult poses not just to the people of Iraq, not just to the people of the Middle East, but to the whole world including to Australia."


Newspaper headline: After Briton’s beheading, PM stops short of airstrikes