The murder of British aid worker David Haines has upped the pressure on London to take the fight to the Islamic State, but Prime Minister David Cameron is proceeding with caution -- and some confusion.

Cameron said Sunday he was ready to "take whatever steps are necessary" to destroy the militant group which has seized territory across Iraq and Syria, and has now killed three Western hostages in less than a month.

But the Conservative leader has yet to commit Britain to join US air strikes against jihadists in Iraq and possibly Syria, despite calls by former generals this weekend warning that IS must be stopped.

Cameron chaired a meeting of the government's emergency Cobra committee to discuss its response to a video released late Saturday showing the beheading of Haines and threatening the life of another British hostage.

In a televised address afterwards, Cameron promised a "calm, deliberate" response, backing the United States' efforts to build an international coalition to take on the jihadists.

"As this strategy intensifies, we are ready to take whatever steps are necessary to deal with this threat and keep our country safe," he said.

He added: "Step by step, we must drive back, dismantle and ultimately destroy ISIL (IS) and what it stands for."

Cameron proved during the 2011 conflict in Libya that he is prepared to authorise targeted air strikes, joining France in a bombing campaign to help rebels overthrow Moamer Kadhafi.

But his calls last year for military action against the Assad regime in Syria ended in humiliating defeat when the proposal was rejected in the House of Commons.

That vote still casts a long shadow, and Foreign Secretary Philip Hammond caused confusion this week when he said that it meant Britain could not act in Syria now against IS.

Hammond was swiftly contradicted by Cameron's Downing Street office, which insisted that nothing had been ruled out.

- 'Fight will take time' -

Cameron said the extremists who killed Haines were the "embodiment of evil" and promised to hunt them down and bring them to justice -- although he said it would take time.

On the broader fight, he highlighted the deployment of British jets to northern Iraq to provide intelligence for the US strikes and the humanitarian aid effort.

The government is arming Kurdish peshmerga forces fighting IS militants in northern Iraq, with the first consignment of heavy machine guns and ammunition announced this week.

And Cameron emphasised the importance of counter-terrorism efforts at home to stop young Britons joining IS and identify those who return with plans to attack Britain.

But his comments are unlikely to satisfy those demanding a show of force from Britain.

"They are just a group of thugs. We have a perfect right to use all means at our disposal to do something about them," the former head of the Royal Navy, Lord West, told Sky News.

The British response had to be "level and sensible", he said, but added: "I think we will have to ... start using very targeted and very carefully used force and all things at our disposal."

The former head of the British army, Richard Dannatt, urged a "decisive" move, while the former commander of British forces in Afghanistan, Richard Kemp, also said it was time for "serious action" against IS.

"If we do not confront and destroy these Islamic State jihadi fighters then their influence will grow, their confidence will grow and the problem will get bigger," Dannatt told Sky News.

But a former military intelligence officer interviewed by the BBC, Frank Ledwidge, warned against doing something that might boost the Islamic State's numbers.

After the rapid advance earlier this year IS was keen to hold on to its territory, he said.

"And there's no better way of doing that than recruiting to it the millions of people in the area who'd be quite happy to fight yet another Western intervention in the area."