A militant group calling itself the Martyrs Brigade said it had kidnapped a missing Western journalist and would kill him if U.S. forces did not leave the holy city of Najaf (search) within 48 hours, the pan-Arab television station Al-Jazeera reported Thursday.

Al-Jazeera showed a video released by the group that depicted a man, who resembled missing journalist Micah Garen (search), kneeling in front of five masked militants, who were armed with rifles and rocket-propelled grenade launchers.

The hostage, who had a mustache, looked down at the ground throughout the video. The authenticity of the tape was not able to be determined.

The sound was not audible, but the announcer said the kidnappers threatened to kill Garen within two days if U.S. forces did not leave Najaf, where they have been fighting with militants linked to firebrand Shiite cleric Muqtada al-Sadr (search) for two weeks.

Garen's father and his fiancee were unavailable for comment when contacted at their homes.

According to witnesses, Garen and his Iraqi translator, Amir Doushi, were walking through a market in the southern city of Nasiriyah (search) on Friday when two men in civilian clothes and armed with Kalashnikov assault rifles seized them, police said.

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

Most of the insurgents behind the kidnappings of scores of foreigners here in recent months have been Sunni Muslims, not those aligned with al-Sadr or other Shiite groups.

When Shiite militants in the southern city of Basra kidnapped British journalist James Brandon (search) on Friday and threatened to kill him, al-Sadr's aides condemned the move and pressured the kidnappers to release him, which they quickly did.

Garen worked for Four Corners media, identified on its Web site as a "documentary organization working in still photography, video and print media."

He has taken photographs as a stringer for The Associated Press and had a story published in The New York Times. His photographs also have appeared in U.S. News & World Report.

Neither U.S. nor Iraqi forces had any word on Garen's fate Wednesday.