A British photojournalist whose abduction had been kept secret for nearly two years appeared as a captive in a video apparently produced by Islamic State.

The video, released Thursday, shows a man who identifies himself as John Cantlie seated at a desk and looking directly into the camera. In a monologue lasting more than three minutes, he briefly describes his abduction, accuses the British government of abandoning him and alludes to forthcoming videos in which he will convey "some facts that you can verify" about...