Prime Minister David Cameron has vowed to hunt down and bring to justice the Islamic State fighters responsible for the killing of a British aid worker.

Cameron's comments on Sunday came after an emergency meeting with his top security advisers in response to an online video that purports to show the beheading of David Haines who was abducted while working in Syria.

"David has been murdered in the most callus and brutal way by an organisation, which is the embodiment of evil," Cameron said.

"The fact that an aid worker was taken, held and brutally murdered at the hand of ISIL sums up what this organisation stands for," Cameron added.

"They are killing and slaughtering thousands of people; Muslims, Christians and minorities across Iraq and Syria. They boast of their brutality. They claim to do this in the name if Islam. That is nonsense. Islam is a religion of peace. They are not Muslims, they are monsters."

Cameron said that the UK could not afford to ignore the threat of IS and would continue to support the international coalition that is fighting the group.

"There is no option of keeping our heads down that would keep us safe… We have to confront this menace. We must drive back, dismantle and ultimately destroy ISIL and what it stands for. We will do so in a clam deliberate way but with an iron determination.

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Fourth hostage threatened

It was the third apparent beheading of a Western hostage in less than a month and an Islamic State member seen apparently killing Haines in the video threatened to execute a fourth, another Briton.

The video blamed Cameron for joining forces with the United States, which has said it is at "war" with the group and is currently hitting it with air strikes in Iraq.

The government in London faces accusations of confusion over its policy towards the Islamic State after conflicting comments from Downing Street and the Foreign Office last week.

During a visit to Berlin, 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.

'Degrade and destroy'

In a statement issued on Saturday night, US President Barrack Obama said the hearts of Americans go out to Haines' family and the people of the United Kingdom.

"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve," Obama said.

"We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world," he said.

In his statement, Obama repeated the pledge he made on Wednesday night in a nationally televised address in which he laid out a strategy to respond to the threat from IS, which is also referred to as ISIL.

Haines' beheading forces UK into war with IS group

"Our objective is clear," he told the nation earlier this week.

"We will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy."

As part of that effort, US Secretary of State John Kerry announced on Saturday that retired Marine General John Allen was joining the State Department as a special presidential envoy to coordinate the international coalition.

The Australian prime minister, Tony Abbot, said on Sunday said his country would  contribute 600 troops and up to 10 military aircraft to the coalition.

Source: Al Jazeera and agencies