LONDON (AFP) - Prime Minister David Cameron said Britain was ready to "take whatever steps are necessary" to destroy Islamic State (ISIS) militants after they murdered a British aid worker, but made no commitment to joining the United States in air strikes.

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 grim-faced premier said in a televised statement from Downing Street.

He continued: "Step by step we must drive back, dismantle and ultimately destroy ISIL (ISIS) 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."

Cameron repeated his support for US air strikes against ISIS 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 ISIS, Cameron made no commitment to British strikes.

London began sending arms this week to Kurdish fighters battling ISIS militants in northern Iraq, but it has faced accusations of confusion over its strategy.

During a visit to Berlin this week, Foreign Secretary Philip Hammond said Britain would not take part in strikes against ISIS 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.

The murder of Haines was revealed in a video released late Saturday, the third beheading of a Western hostage in less than a month.

In the footage, a hooded militant blames Cameron for joining forces with the United States and says the alliance will drag the British people into "another bloody and unwinnable war." Cameron described 44-year-old Haines as a "British hero", saying that "his selflessness, his decency, his burning desire to help others has today cost him his life".

Those who killed him "are not Muslims, they are monsters".

Haines had been taken hostage in Syria in March 2013 while working for a French NGO.