LONDON (AFP) - Prime Minister David Cameron will gather some of his top security advisors on Sunday to discuss Britain's response to an online video showing the murder of an aid worker by Islamic State in Iraq and Syria (ISIS) militants.

He will chair a meeting of the government's emergency response committee, Cobra, attended by senior representatives of the military and security services, the Foreign Office and Home Office interior ministry, a spokeswoman said.

Cameron has condemned the beheading of 44-year-old David Haines as "pure evil", after the video was released late Saturday showing his execution by the ISIS jihadist group, which has swept across Iraq and Syria in recent months.

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

It was the third apparent beheading of a Western hostage in less than a month and the militant 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 jihadists and launched air strikes against them in Iraq.

The government in London faces accusations of confusion over its policy towards IS 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.