As Shiite groups, some backed by Iran, escalate their demands for political control in a new Iraq, the Bush administration appears to have little influence or contacts with the factions that could pose the biggest obstacle to creating a pro-American democratic government, U.S. officials and independent analysts said yesterday.

Even so, U.S. officials expressed confidence that the turmoil is temporary. "The Iraqi people will do something about it," Defense Secretary Donald H. Rumsfeld said. "If they don't want those people in and those people don't subscribe to the principles that we've set forth . . . then they'll stay out, and that's life."

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."

Iraq's most important Shiite group, the Supreme Council for the Islamic Revolution in Iraqi, (SCIRI), yesterday boycotted the first U.S.-sponsored meeting of Iraqi political and religious leaders in the town of Ur to discuss the country's political future. The organization cited as its objection "a process which is under an American general" -- retired Lt. Gen. Jay M. Garner, head of the Office of Rehabilitation and Humanitarian Assistance, which is overseeing the administration's postwar effort in Iraq. SCIRI is based in Iran and has close ties to the Iranian government.

In Nasiriyah, thousands of Iraqi Shiites protested the talks, saying that key Shiite groups and their leaders were not included in the meeting. In Kut, hundreds of protesters blocked U.S. Marines from entering city hall to meet with a radical anti-American Shiite cleric who had declared himself in control of the city.

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.

"It's a mess," a senior administration official said.

Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, warned Iran yesterday not to allow the Badr Brigade, SCIRI's 10,000-member Shiite Iraqi army based in Iran, to enter Iraq. "They should not be inside Iraq," he said. Rumsfeld delivered a similar warning on March 31 after reports that members of the brigade had crossed into northern Iraq.

U.S. intelligence analysts and Iraq experts said they warned the Bush administration before the war about the clashes that could result if the Baath Party vanished and nothing replaced it. "This comes as no surprise," the administration official said.

"The U.S. has limited influence over the battle within the Shiite community, because a lot of the battles are being fought out in mosques in Shiite areas where the U.S. is hesitant to have a presence," said Kenneth Katzman, chief Middle East analyst at the Congressional Research Service. "It will be hard to stop this."

Iraqi Shiite factions, some affiliated with Iran, are competing for influence in Iraq. Because Hussein had brutally suppressed Iraqi Shiites for so many years -- executing hundreds of clerics and activists and exiling thousands more -- few Shiite civic or religious leaders of national standing survived. Many of the leading figures are the sons of respected Shiite clerics from earlier decades, some of whom had been jailed or murdered by Hussein.

The U.S. government tried for months to make common cause with SCIRI, which fought with Iranian forces against Iraq in the Iran-Iraq war during the 1980s. Those ties engender deep skepticism about SCIRI on the part of many Iraqis, including some Shiites.

In December, President Bush listed SCIRI in a presidential directive as one of the Iraqi opposition groups that would split $92 million to organize the fight against Hussein. SCIRI also was the main Shiite group that attended a U.S.-backed Iraqi opposition conference that month in London. A top SCIRI leader visited Washington to plan anti-Hussein activities before the war.

But in recent months, U.S. attempts to work with SCIRI came to naught. Now SCIRI is expressing bitter antipathy toward the United States, which it calls "colonialist" for its intervention in Iraq.

SCIRI is led by Mohammed Bakir Hakim, whose deceased father was a leading ayatollah in Iraq. Hakim has said he plans to move back to Iraq from Iran.

Another Shiite group is Dawa Islamiyah, or Islamic Call, which has several thousand fighters under arms. Dawa agents almost succeeded in killing Uday Hussein, the deposed Iraqi president's eldest son, in 1996, shooting him 14 times as he drove in Baghdad. Dawa is split into factions, some of which are based in Iran.

Another leading faction is composed of the devotees of the Grand Ayatollah Mohammed Sadiq Sadr, who was executed by Iraqi security forces in 1999 for organizing opposition to the Baathists. He had also criticized what he called the "sleeping clerics" of Iraq who failed to oppose Hussein. Now his followers are trying to rename Saddam City, the squalid, predominantly Shiite suburb of Baghdad, Sadr City after the deceased cleric.

His son, Muqtada, 22, is trying to gain control of key sites in the holy Shiite city of Najaf, and is threatening other leaders who have influence there.

Sadr, who is vehemently anti-American, has said he opposes mainstream religious leaders such as Sistani, whom he ordered briefly imprisoned in his home a few days ago. Sistani, Iraq's leading Shiite cleric, was jailed on occasion by Hussein but generally avoided showdowns with the Baathists. Iraq experts said Sadr is angling to become a leader in the post-Hussein Iraq.

Iraqi Muslim Shiites gather in Karbala to hear a sermon. Shiites make up about 60 percent of the Iraqi population but have been repressed for decades.Abdul Majid Khoei, a Shiite cleric who was based in London, talks to British Prime Minister Tony Blair in 2001. Khoei was recently stabbed to death.Grand Ayatollah Mohammed Sadiq Sadr, a Shiite leader, was executed by Iraqi security forces in 1999.