The Islamic State jihadist group says it has executed British aid worker David Haines.

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

It says the execution is in retaliation for British prime minister David Cameron entering a coalition with the United States against the militants.

This would be the third such execution in recent weeks, after two US journalists taken hostage in Syria were shown being murdered.

Haines was abducted in Syria in 2013 while working for an international aid agency.

Until recently the British government had managed to keep his kidnapping secret out of concern for his safety until the most recent IS video identified him as a captive.

Earlier, the family of Mr Haines issued a public plea urging his captors to contact them.

In a short statement released through Britain’s Foreign and Commonwealth Office late on Friday, the family said: “We are asking those holding David to make contact with us.”

Islamic State militants have beheaded two American journalists and posted video evidence online. At the end of the last video, they threatened to kill Haines next and briefly showed him on camera.

Haines was abducted in Syria in 2013 while working for an international aid agency.

The British government had managed to keep his kidnapping secret out of concern for his safety until the most recent IS video identified him as a captive.

The Foreign Office said in a statement that the extremists had put Haines’ name in the public domain. The statement said the hostage’s family would not make further comments.