The Islamic State jihadist group claimed Saturday it beheaded British aid worker David Haines, in what would be the third such execution in recent weeks, after two US journalists were shown murdered.

The Islamist group released a video, available on the website of private terrorism monitoring group SITE, purportedly showing a masked militant beheading Haines.

The two-minute-27-second video titled "A Message to the Allies of America" blames British Prime Minister David 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.

"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," the executioner says in the video.

The militant, who may be the same man as in the previous videos, told 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 British hostage.

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. (AFP)