The United Nations is closing in on a slate for the new Iraqi government, with a Shiite nuclear scientist who spent years in the notorious Abu Ghraib prison emerging as the leading candidate for prime minister, according to Iraqi and U.S. officials.

U.N. envoy Lakhdar Brahimi and Robert D. Blackwill, the U.S. presidential envoy to Iraq, are still working out the "complicated geometry" of dividing power among Iraq's disparate ethnic and religious factions, a senior administration official in Baghdad said yesterday. But Brahimi has met several times this month with Hussain Shahristani, who said in an interview yesterday that if asked, he would reluctantly accept the post of prime minister in Iraq's first post-Saddam Hussein government.

"If they consider my participation essential, I'll try to convince them otherwise," said Shahristani, who was educated in London and Toronto. "But if they're not convinced and they ask me to take a role . . . I cannot refuse. I must serve my people."

U.S. officials say that although negotiations have not been concluded for the 30 jobs -- prime minister, the ceremonial positions of president and two vice presidents, and 26 cabinet ministries -- Shahristani has emerged as by far the most attractive of a few candidates to lead the caretaker government after June 30.

"The game has not played out yet, but Shahristani is the candidate to beat," a senior State Department official said. An Iraqi who has long known Shahristani called him "a captain of men," even though he has limited political experience.

Despite the first indication of headway in forming a new government, the United States still faces potentially serious differences with allies and members of the U.N. Security Council critical to passage of a new resolution endorsing President Bush's five-point plan for Iraq. The most divisive issue is proving to be the power of foreign forces that remain after the occupation ends June 30. The current draft stipulates that the U.S.-led force will have "authority to take all necessary measures" to ensure security and stability in Iraq.

British Prime Minister Tony Blair, the most stalwart U.S. ally in Iraq, said yesterday that the interim Iraqi government should have veto power over foreign military operations. "If there is a political decision as to whether you go into a place like Fallujah in a particular way, that has to be done with the consent of the Iraqi government," Blair told a news conference. "The transfer of sovereignty has to be real and genuine."

But in Washington, Secretary of State Colin L. Powell said U.S. troops will do what American commanders deem necessary. "If it comes down to the United States' armed forces protecting themselves or in some way accomplishing their mission in a way that might not be in total consonance with what the Iraqi interim government might want to do at a particular moment in time, U.S. forces remain under U.S. command and will do what is necessary to protect themselves," Powell told reporters.

Powell dismissed suggestions of fallout between the allies over a new Iraqi government, noting that coordinating committees will be set up to deal with Iraqi "desires, wishes and feelings" and the kind of issues that the United States has faced with troop deployments in countries in Europe and Asia over the past half-century.

In a telephone call yesterday from President Jacques Chirac to Bush, France called for changes in the draft U.N. resolution to ensure that Iraq's new government inherits genuine sovereignty and "real change," according to a statement by Chirac's office. France, which led opposition to the war against Hussein, also wants the caretaker Iraqi leadership to be involved in approving military operations -- and for the government scheduled to be elected early next year to be able to determine whether foreign forces remain in Iraq.

The White House played down the differences. "I had a great conversation with President Chirac," Bush told reporters. "We share the same goal: a free and stable and peaceful Iraq."

But the issue is almost certain to remain contentious. Shahristani said decisions about the use of military force should be made by a sovereign Iraqi government, not solely by U.S. commanders.

"I really think it's the Iraqis who will know best how they should deal with the situation," he said in an interview. "The decision should be made by the Iraqis -- of course, after consultations with the coalition."

Shahristani said he supports the continued presence of international troops in Iraq to help the country's fledgling security forces deal with insurgents and terrorists. "We will have to discuss how that [international] force will operate in conjunction with Iraqi forces on the ground," he said.

Shahristani said leading the interim government would be an "extremely difficult job." The caretaker administration, he said, will need to focus primarily on addressing security issues and preparing for national elections in early 2005. "We've been hearing about holding elections for some time now, but we have yet to see any real preparation on the ground," he said.

The interest by U.N. and U.S. envoys in the 62-year-old nuclear scientist reflects their goal of crafting a government with broad legitimacy both at home and with the international community and reaching beyond the 25 men and women appointed to the Governing Council last year, who have failed to win widespread support among Iraqis.

Shahristani, who has a doctorate in nuclear chemistry from the University of Toronto, served as chief scientific adviser to Iraq's atomic energy commission until 1979, when Hussein became president. When he refused to shift from nuclear energy to nuclear weaponry, he was jailed. For most of a decade, he was in Abu Ghraib prison, much of it in solitary confinement. He escaped in 1991 and fled with his wife and three children to Kurdish-controlled northern Iraq and, eventually, Iran, where he worked with Iraqi refugees. He later moved to Britain, where he was a visiting university professor.

But unlike other exiles, Shahristani was not active in opposition parties, choosing instead to focus on humanitarian aid projects. He does, however, have a critical connection: He is close to Grand Ayatollah Ali Sistani, the country's most powerful Shiite cleric, whose support is essential for the viability of an interim government.

Shahristani, who has described himself as an adviser to Sistani, said he has met with the ayatollah several times since the fall of Hussein's government. Shahristani said Sistani has played a "very, very constructive" role in Iraq over the past year. Iraqi officials familiar with Brahimi's mission said Shahristani's lack of political affiliation could be an asset, allowing him to serve as a bridge between various factions.

Shahristani crossed into Iraq two days before Hussein fell to deliver aid to the city of Karbala. Since then, he has divided his time between Karbala and the southern port of Basra, working on humanitarian projects in both places.

"I've been actively working to help the Iraqi people to free themselves from Saddam's tyranny, but I have always concentrated on serving the people and providing them with their basic needs rather than party politics," he said.

Iraqi officials familiar with Brahimi's mission said it was an op-ed piece Shahristani wrote for the April 29 Wall Street Journal that piqued Brahimi's attention. Headlined "Election Fever," the piece criticized the U.S. occupation authority for failing to prepare for elections sooner and for promulgating an interim constitution that was drawn up behind closed doors. He called for the government taking power on June 30 to have limited powers aimed at preparing the country for elections -- a position advocated by Sistani.

Brahimi is now expected to announce the full interim government lineup Tuesday, said the senior administration official in Baghdad. The U.N. and U.S. envoys are still trying to negotiate a compromise with Kurdish leaders, who are demanding that they be given the presidency or premiership.

A senior Kurdish politician, speaking on the condition of anonymity, said the Kurds had been offered two of the four most powerful ministries in exchange for accepting one of the two vice presidential posts, but Kurdish leaders have not decided whether they will accept the proposal. "We're still negotiating," the politician said. "We have not made a decision."

Chandrasekaran reported from Baghdad. Staff writer Colum Lynch contributed to this report from the United Nations.