A politically independent Shiite Muslim who had been a top choice of the United States and the United Nations to become Iraq's prime minister withdrew from consideration after objections from formerly exiled Shiite politicians who want the job for themselves, officials involved in the political transition said Thursday.

The politicians' refusal to accept Hussain Shahristani as prime minister has complicated U.S. and U.N. efforts to form an interim Iraqi government to assume limited political authority on June 30, forcing U.N. envoy Lakhdar Brahimi and top U.S. officials to scramble for new candidates. The U.S. occupation authority had hoped to have the government named by Monday, to give appointees a month to work into their new jobs, but U.N. officials said that goal now appears unattainable.

The stand against Shahristani also struck a serious blow to attempts by the United States and the United Nations to fill top positions in the interim government with independents and technocrats instead of politicians, many of whom spent years in exile and enjoy little public support.

The U.S. government funded many exiled opposition politicians during the rule of President Saddam Hussein, and several were appointed to Iraq's Governing Council after Hussein was toppled last year. Because of their unpopularity, however, the occupation authority has sought to minimize their role in Iraq's next government. Yet Shahristani's inability to win their approval illustrates their continuing ability to disrupt U.S. plans for the country's political transition.

"They feel they are a kind of club, and this was a person who is outside their club," said an Iraqi official close to Shahristani. The official said Shahristani, who had met with Brahimi several times over the past few weeks and was regarded as the U.N. envoy's top choice, talked to Brahimi on Tuesday night "and said he couldn't be a candidate because he cannot get the support of this club."

The Shiite politicians who opposed Shahristani's appointment included Ahmed Chalabi, a onetime U.S. ally who heads the Iraqi National Congress, which had been funded by the Pentagon; Ayad Allawi, the leader of the Iraqi National Accord, which had been supported by the CIA; Adel Abdel-Mehdi of the Supreme Council for the Islamic Revolution in Iraq; and Ibrahim Jafari of the Dawa party, according to people familiar with the discussions with Brahimi.

Allawi, Abdel-Mehdi and Jafari each wants to be prime minister, the sources said.

The process of forming the interim government has also been complicated by demands from ethnic Kurdish politicians. They want a Kurd to be given either the presidency or the prime minister's post. Brahimi and the top two U.S. officials involved in forming the government -- L. Paul Bremer, the civilian administrator of Iraq, and Robert D. Blackwill, a presidential envoy -- have offered the Kurds, who account for about 20 percent of the country's population, one of two vice presidential posts and two of the four most powerful cabinet ministries.

According to sources close to the process, Brahimi, Bremer and Blackwill had planned to give the jobs of president and prime minister to Arabs -- the presidency to a Sunni and the prime ministership to a Shiite. The Shiites, who make up about 60 percent of the population, also would receive one of the vice presidential slots.

Kurdish leaders, who appeared close to accepting the U.S. and U.N. offer a few days ago, have renewed their insistence on one of the top posts, Kurdish officials said.

"The Kurds need one of those two top positions," a senior Kurdish politician said. "We must not be marginalized and must not appear [to be] second-class citizens."

The demands of the Kurdish leaders and the refusal of the Shiite politicians to accept an independent prime minister have turned the process of forming the interim government, which is supposed to be a caretaker administration in power for only seven months, into a complicated, high-stakes negotiation among Iraqis, U.S. officials and U.N. diplomats. Brahimi, Bremer and Blackwill have spent the past few weeks meeting with dozens of different groups of Iraqis in an effort to forge a consensus.

"What's happening here is the fully blown breakout of politics," a senior Bush administration official said.

To some people involved in the process, though, it appears that a handful of former exiles are once again forcing a revision of American plans. Opposition from Iraqi political leaders scuttled earlier U.S. initiatives to write a permanent constitution before the handover of authority and to select a transitional administration through caucuses.

"The exiled politicians are demanding a dominant role, and they may get it just because there isn't much time left and nobody wants to risk people boycotting the new government," an official involved in the process said.

Brahimi had favored the idea of giving the presidency and the two vice presidential jobs to political leaders while restricting the prime ministership and cabinet posts to people with technical expertise. But faced with demands from political leaders for a greater role in government, the U.N. envoy may be forced to revise his blueprint, people with knowledge of the process said.

Shahristani, 62, a nuclear scientist who was imprisoned by Hussein's government for more than a decade after he refused to help Iraq build nuclear weapons, has little political experience. Shahristani escaped Iraq in 1991, but unlike many other Iraqis who lived in exile, he was not active in opposition political parties, choosing instead to focus his energies on helping Iraqi refugees. For the past year, he has avoided politics and worked on humanitarian aid projects in southern Iraq.

"With all respect to Mr. Hussain Shahristani, this new situation needs somebody who is known by the street, someone who has a political platform and someone who has demonstrated excellent performance during the last year," said Adnan Ali, a senior adviser to Jafari. "Dr. Shahristani is a nuclear expert who has a long history of being a prisoner and a long history of working with charities. It's difficult to see how he can jump from this kind of specialty to deal with sovereignty and the ministries and the budget."

Ibrahim Janabi, a spokesman for Allawi, said Shahristani "will find it difficult to solve the problems that Iraq faces now" because he does not have a political background. "We need people with political skills at this time."

Speaking at a news conference in Baghdad, Shahristani said he had informed Brahimi that he did not wish to serve in the interim government and instead wanted "to continue to serve the Iraqi people through my current activities in the humanitarian field." He would not say publicly why he had asked not to be considered other than to say his reasons "were not personal at all."

"I do believe there are convincing reasons, at least they are convincing to me," he said. "But I don't think it's correct to discuss these issues at such a sensitive time in the history of the country. I'd rather pave the way with my silence for a new Iraqi government to try to lead the country on the road to democracy."

Shahristani said the new prime minister should be someone "who is not a member of one of the leading parties." The job, he said, should go to the "most competent person."

"I think if the Iraqis could work together in a better way, we would have a better chance of improving the process," he said.

Shahristani did not rule out a future in politics, though. He said he would run for a seat in parliament when national elections are planned early next year.

Staff writer Robin Wright in Washington contributed to this report.

Iraqis demonstrate in Baghdad in support of a tribal leader they want to be appointed to the interim government after Hussain Shahristani, a political independent and a favorite of U.S. and U.N. officials, withdrew as a possible premier.