A group calling itself the Martyrs Brigade said it had kidnapped a missing American journalist and would kill him if U.S. forces did not leave Najaf within 48 hours, the al-Jazeera satellite television network reported Thursday.

The network broadcast a video that depicted a man resembling the missing journalist, Micah Garen, kneeling in front of five masked men armed with rifles and rocket-propelled grenade launchers. The hostage, who had a mustache, looked down throughout the video.

It was not possible to verify the authenticity of the tape.

A voice on the tape said the kidnappers would kill Garen, who also has French nationality, within two days if U.S. forces did not leave Najaf, where they have been fighting the Mahdi Army militia of radical Shiite cleric Moqtada Sadr.

After a British reporter was kidnapped last week in the southern city of Basra, aides to Sadr condemned the hostage-taking and pressured the kidnappers to release him.

Garen and his Iraqi interpreter, Amir Doushi, were seized by two armed men in civilian clothes in the southern city of Nasiriyah on Friday, said police, citing witness accounts.

At the time of his abduction, Garen, 36, was working on a story about the looting of archaeological sites in Iraq, according to his fiancee, Marie-Helene Carleton.

Garen worked for Four Corners Media, identified on its Web site as a "documentary organization working in still photography, video and print media." He also has worked as a freelance photographer for the Associated Press. An article he wrote about looting of antiquities was published in the New York Times in April.