The Islamic State group has released a video purporting to show the beheading of British aid worker David Haines - an act described by the British prime minister as "pure evil".

The video, released late on Saturday, shows Haines, 44, being killed in a desert location by a masked man. The father of two children, from Perth in Scotland, was abducted last year while working for the French aid agency Acted.

The video comes after the murder of James Foley and Steven Sotloff, both American journalists, in similar settings and manners.

The masked man in the latest video states Haines was killed because the UK's prime minister, David Cameron, had promised to arm Kurdish Peshmerga fighters in Iraq against the Islamic State fighters.

"This British man has to pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State."

The UK policy was announced earlier this week.

At the end of the video, another hostage was shown and the masked man said he would be killed if Cameron continued to support the fight against the Islamic State group, also known as ISIL.

The video was released a day after Haines' family appealed to the Islamic State group to free him.

Cameron said in a statement early on Sunday: "This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil.

"My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal."

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

US President Barack Obama condemned the "barbaric murder" and said his country stood shoulder to shoulder "with our close friend and ally in grief and resolve".

Mike Haines, David Haines' brother, said his brother was devoted to humanitarian work. "David was most alive and enthusiastic in his humanitarian roles.

"His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair. He was and is loved by all his family and will be missed terribly.''