The day after the U.N. Security Council unanimously granted the United Nations a "leading role" in building democratic institutions in Iraq, wary officials of the world body were asking one another what will happen if bombs continue to explode.

A deadly terrorist attack on its headquarters drove the United Nations from Iraq last August, forcing the organization to operate largely from outside the country and shaking the confidence of U.N. staff. Now, the Security Council has set a task that some U.N. officials said they will accept more from duty than desire.

"There is enormous trepidation at every conceivable level. We're not even at the point where we have secure facilities for Iraqi staff," said one well-connected U.N. worker who explained that the organization also faces formidable logistical challenges. "There's a real question about how fast the U.N. can respond."

Secretary of State Colin L. Powell committed the Bush administration to establishing or supporting a force of 4,000 to 5,000 troops to protect U.N. workers. A senior U.N. official said Wednesday that no countries have yet agreed to take up arms on the organization's behalf.

The bulk of the United Nations' foreign staff will continue to be based outside Iraq for safety reasons, said a senior adviser to Secretary General Kofi Annan. The organization expects to depend more than usual on approximately 600 Iraqi employees able to work less obtrusively than foreigners who have been targeted by guerrillas.

The effectiveness of the United Nations in bridging Iraqi political divides and helping to stage national elections by the end of January will depend, said another top official, "on how the security situation evolves and whether the spoilers try to spoil. Security is the major constraint."

The reaction of U.N. staff members after Tuesday's vote illustrates the difficulties ahead as the U.S. administration seeks to broaden international participation in Iraq's transition. Mindful of reluctance abroad to send personnel to Iraq, the White House hopes the imminent transfer of limited authority to Iraqis will combine with U.S. military operations to quiet a violent insurgency.

Meanwhile, as U.N. planners studied the implications of the council's endorsement of the interim Iraqi government and the beginnings of a U.S. exit strategy, officials in Baghdad and Washington sought Wednesday to extinguish a brush fire over Iraq's constitution.

Kurdish and Shiite Arab leaders are arguing over a section of Iraq's interim constitution, approved in March, that allows voters in any three provinces to nullify a permanent constitution by a two-thirds vote. The provision, designed to protect the Kurdish minority, is considered unfair by some Shiite politicians who fear being handicapped by a Kurdish veto.

Jalal Talabani and Massoud Barzani, Iraq's two most prominent Kurdish politicians, warned in a letter to President Bush this week that Kurds would refuse to participate in the central government if the Shiites do not honor the March agreement. Two weeks ago, the Kurds similarly threatened to pull out of the interim government unless they received more prominent cabinet assignments. The tactic succeeded.

At a news briefing in New York, U.N. special envoy Lakhdar Brahimi said both sides have legitimate concerns. He expressed confidence that the Kurdish leaders want to be "full partners" in Iraq's future and said they deserve a promise that their rights as a minority will be respected. He said the Shiites, by the same token, are right to want an assurance that Kurds seeking autonomy will not deliver a blanket veto.

After nearly a year when the United Nations was forced to limit its presence in Iraq to protect the safety of its workers, the Bush administration's newfound willingness to relinquish control and the Security Council's passage of the latest resolution appears likely to create a larger U.N. role.

As Brahimi told reporters, "The United Nations cannot say no to helping the Iraqis recover their sovereignty, no matter how difficult, no matter how complicated. . . . We accept those risks if the work we are asked to do is important enough to warrant the risks."

That issue was much debated in the corridors of the United Nations after the Aug. 19 bombing, which killed U.N. envoy Sergio Vieira de Mello and 21 others. The attack shocked the world body, which had focused on humanitarian deeds after the Security Council refused to endorse the U.S.-led invasion. Feelings ran high that U.N. employees were enduring too much danger in return for marginal influence.

With that equation shifting, one challenge for the United Nations is to define a role that has long been vague. The principal effort laid out in the resolution is to help assemble a political conference to be held in July and prepare for elections to a national assembly by the end of January.

U.N. experts are also expected to help plan a comprehensive census, advise on the drafting of a permanent constitution and offer wisdom on government services, judicial reform and human rights. Money is not a problem, U.N. officials said, but the ability to put together an effective staff and operate inside Iraq remains to be seen. One worry in the corridors is that the Security Council's expectations are unrealistic.

The Annan adviser reported that even some Iraqi staff members have been threatened and have moved from their homes to locations they consider safer.

"I don't think we put a lot of people in there," he said.

Staff writer Robin Wright in Washington contributed to this report.