The beheading of a British hostage by Islamic State militants was an "act of pure evil," Prime Minister David Cameron said Sunday, vowing Britain will do all in its power to bring the killers to justice.

"This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil," he said in a statement released by his Downing Street office.

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

Cameron, who was seen entering Downing Street shortly before midnight Saturday, will chair a meeting of the government's emergency Cobra committee early Sunday in response to an online video purportedly showing a masked militant killing Haines.

The video appeared less than a day after the aid worker's family on Saturday appealed to his captors to contact them.

Prime Minster Cameron tweeted that his "heart goes out to his family who have shown extraordinary courage and fortitude."

Britain's Foreign Office said they were "working as quickly as we can to try and verify" the video.

Two US journalists have been murdered in similar circumstances in recent weeks.

The latest two-minute-27-second video titled "A Message to the Allies of America" blames 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 video opens with a clip of Cameron describing the British strategy of working with the Iraqi government to help arm Kurdish fighters against "these brutal extremist militants," and to offer aid, diplomacy, and military help to pressure IS.

Haines, dressed in an orange jumpsuit, identifies himself in English and calmly explains he is paying the price for Cameron's policy.

Addressing Cameron, the executioner also speaks in English, with an apparently British accent.

"You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor Tony Blair did, following a trend amongst our British prime ministers who can't find the courage to say no to the Americans."

Video piles pressure on Cameron

The militant, who may be the same man as in the previous two beheading videos, tells Britain the alliance will "accelerate your destruction" and will drag the British people into "another bloody and unwinnable war."

He also threatens to execute another captive, identified in a caption by name as another British citizen.

Scottish-born Haines, 44, was taken hostage in Syria in March 2013 and was threatened in a video released this month depicting the beheading by an IS militant of the US journalist Steven Sotloff.

Haines had been working for the Agency for Technical Cooperation and Development (ACTED), an international relief charity, and was previously involved in humanitarian work in the Balkans, parts of Africa and elsewhere in the Middle East.

Sotloff and fellow US journalist James Foley had also been kidnapped in Syria. IS released a video claiming Foley's execution on August 19, and Sotloff's two weeks later on September 2.

In a statement issued by the Foreign Office in London earlier Saturday, Haines' family had appealed to his captors to contact them, saying the extremists had not responded to any of their attempts at communication.

"We are the family of David Haines. We have sent messages to you to which we have not received a reply. We are asking those holding David to make contact with us," the family said.

The video is certain to intensify calls in Britain for Cameron to consider military action against IS in Iraq, and possibly even in Syria, but the prime minister will be wary of playing into the hands of the captors and escalating tensions.

He is also recovering from last year's humiliation of failing to achieve parliamentary support for air strikes against Syria after President Bashar al-Assad's was found to have used chemical weapons against rebels.

The videos also raise the issue of ransoms, which Britain and the US refuse to pay, arguing it would further endanger their citizens.

In recent days, the families of both Foley and Sotloff have complained in interviews that US officials told them they would prosecuted if they raised ransoms to release the men.