Prime Minister David Cameron chaired a meeting of the government's emergency response committee today under growing pressure to sanction air strikes after an Islamic State video showed the beheading of a British hostage.

Footage of the murder of David Haines by IS militants fighting in Iraq and Syria means Cameron, who is also trying to persuade Scotland to reject independence in a referendum on Thursday, is under pressure to get much tougher with IS.

He has said he isn't ruling out any options to degrade IS, with the exception of putting boots on the ground, but is facing increasingly loud calls from some of his own Conservative lawmakers and from former military chiefs to join the United States in launching air strikes.

Cameron's last attempt to get the British parliament to back such air strikes, against Syria last year, ended in failure when lawmakers voted against such action.

With Scotland his domestic priority, he is aware that many Scots have traditionally been more skeptical of British military action overseas and that proposing air strikes now could risk alienating them before Thursday's independence vote.

Cameron, who returned to London ahead of schedule on Saturday night to chair the emergency meeting, has called the murder of Haines, a 44 year-old Scottish aid worker, an act of pure evil and vowed to bring his killers to justice.

"This is a despicable and appalling murder of an innocent aid worker," he said in a statement on Saturday.

"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."

Britain's Foreign Office said the video showed "all signs" of being genuine. Sources could not immediately verify the footage, but the images were consistent with those of the filmed executions of two American journalists, James Foley and Steven Sotloff, in the past month.

Haines's executioner appears to be the same man who featured in videos with Foley and Sotloff. The man, nicknamed "Jihadi John" by Western media, seems to have a British accent. At the end of the same video, another hostage is shown and threatened.

Security services in Britain have been trying to identify the executioner. A British security source speaking on condition of anonymity said an investigation was underway into the killings and that senior intelligence officials had attended the meeting of the COBR emergency committee that Cameron chaired.

The source declined to go into detail about what, if any, progress the investigation had made.

AIR STRIKES

Faced with the rise of IS, Britain has so far confined itself to delivering humanitarian aid, carrying out surveillance, arming Kurdish forces who are fighting IS militants and promising training in Iraq.

On military action, London supports U.S. air strikes while keeping its own options open.

Sir Richard Dannatt, former head of the British army, said on Sunday IS executions should not deter the government from taking military action against the militants.

"If we don't confront and destroy these Islamic State Jihadi fighters then their influence will grow, their confidence will grow and the problem will get bigger," he told sources.

Mike Haines, brother of the slain aid worker, said on Sunday that David had chosen humanitarian work but had been murdered in cold blood.

"He was and is loved by all his family and will be missed terribly," Mike Haines said in a statement released through the British Foreign Office. He said his brother left behind two daughters from two marriages.

Alex Salmond, Scotland's First Minister, told sources Haines's murder was an "unspeakable act of barbarism."

Asked in a sources if an independent Scotland would be prepared to take military action against IS, he said any response must be done under United Nations auspices, underlining Scottish anxiety about the prospect of unilateral military action.

"You can't have a strategy where you bow to terrorism. There's an urgent requirement to get back to collective (action) under the United Nations," he said.

Salmond has called the 2003 Iraq invasion illegal because it was not launched with U.N. approval.

Haines was remembered in prayers at the morning service in Edinburgh's St. Mary's Cathedral, where provost Graham Forbes praised him for his dedication to humanitarian work.