CNN is reporting that a third hostage execution video has been released by the Islamic State.

On Saturday, a video was posted to a website associated with the terror group claiming to show the beheading of British aid worker David Haines.

The militant says in that video, “We take this opportunity to warn those governments who’ve entered this evil alliance of America against the Islamic State to back off and leave our people alone.”

British officials said after the video’s release that they had sent troops to try to rescue an unidentified British citizen “some time ago,” but failed. They released no other details.

ISIS, which also calls itself the Islamic State, has killed thousands in Syria and Iraq as it presses a military campaign to establish itself as an Islamic caliphate. Witnesses report mass killings, beheadings and crucifixions.

Haines was kidnapped in March 2013 near a Syrian refugee camp. Intelligence officials are currently working to authenticate the video.