Prime Minister David Cameron will gather some of his top security advisers to work out a response to an online video that purports to show the beheading of a British aid worker by Islamic State fighters.

Cameron will chair a meeting of the government's emergency response committee, Cobra, on Sunday, which will be attended by senior representatives of the military and security services, a spokeswoman said.

The prime minister described the  beheading of 44-year-old David Haines as "pure evil", after the video was released late on Saturday showing his apparent killing by the Islamic State group, which has swept across Iraq and Syria in recent months.

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

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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 Barack 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 the Islamic State group, 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 counterterrorism 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.

Australian Prime Minister Tony Abbott said on Sunday his country would  contribute 600 troops and up to 10 military aircraft to the coalition.

Source: Al Jazeera and agencies