Many Iraq experts, including some who advise U.S. intelligence and military officials, said the demands by competing Iraqi Shiite groups in the southern cities of Najaf, Nasiriyah, Kut and Karbala, as well as in parts of Baghdad, are the beginning of a serious challenge to U.S. efforts to bring about a pro-Western democracy. Shiites make up about 60 percent of the Iraqi population but have been repressed for decades by Saddam Hussein's Sunni-dominated Baath Party.

"U.S. officials don't have a lot of traction in the Shiite community in Iraq," said A. William Saami, a Middle East expert for Radio Free Europe/Radio Liberty who has advised U.S. national security agencies. "This is going to get worse before it gets better. . . . U.S. government officials are cognizant of these issues but don't understand them."

Last week, a London-based Shiite cleric who was working with U.S. forces, Abdul Majid Khoei, was stabbed to death at a shrine in Najaf, apparently by followers of a young, anti-American Shiite leader. The same group, led by Muqtada Sadr, also surrounded the Najaf home of the nation's top Shiite cleric, Ayatollah Ali Sistani, and ordered him to leave the city before being persuaded by tribal elders to disperse.