SANA, Yemen — Seething over an attempt last month by United States commandos to free an American hostage, Al Qaeda’s Yemeni affiliate has threatened to kill him by the end of the week and warned the Obama administration against further “foolish action,” according to a video posted on YouTube.

The journalist, Luke Somers, who appears in the video posted late Wednesday, has been missing since September 2013, when he was snatched off a street in Sana, the Yemeni capital.

Mr. Somers, speaking to the camera, appears composed and healthy, wearing a purple shirt and glasses.

“I’m looking for any help that can get me out of this situation,” he said. “I’m certain that my life is in danger. So as I sit here now, I ask if there’s anything that can be done, please let it be done.”