UNITED NATIONS - The voice was soft, calm, familiar. But the scenario Kofi Annan sketched out was chilling.

A nuclear bomb goes off in a great city. Chaos ensues, and a frightened world asks, "Was this an act of terrorism? Was it an act of aggression by a state? Was it an accident?"

Tens or hundreds of thousands would be dead, the U.N. chief said, and questions, implications and dread would consume world leaders. Treaties might collapse, trade and economies totter, human rights and freedoms come under threat. And statesmen would ask: "How did it come to this?"

It was Monday's arresting opening to a month-long conference reviewing the workings of the Nuclear Nonproliferation Treaty, at a moment of rising nuclear tensions in the world, on a day when speakers called for concessions from many sides - Iran, North Korea, America, Russia - to move toward a world free of the nuclear threat.

"Ultimately, the only way to guarantee that they will never be used is for our world to be free of such weapons," Annan said.

But U.S. delegation chief Stephen Rademaker made clear the concessions Washington was most interested in would come from Iran, accused by the Americans of using the cover of a nuclear-power program to plan the building of weapons.

"The treaty is facing the most serious challenge in its history," the assistant secretary of state told delegates from more than 180 nations. "We must confront this challenge."

Under the 35-year-old Nuclear Nonproliferation Treaty (NPT), states without nuclear arms pledge not to pursue them, in exchange for a commitment by five nuclear powers - the United States, Russia, Britain, France and China - to move toward nuclear disarmament. Three other nuclear states - Israel, India and Pakistan - remain outside the treaty.

The NPT is reviewed every five years at conferences whose consensus positions give valuable political support to nonproliferation initiatives. At the 2000 meeting, the nuclear powers committed to "13 practical steps" toward disarmament, but critics complain the Bush administration - by rejecting the nuclear test-ban treaty, for example - has come up short.

"We are greatly disappointed" by "unsatisfactory progress" toward disarmament, said New Zealand's Marian Hobbs.

Rademaker sought to focus attention instead on Iran, saying, "We dare not look the other way."

The Iran question hinges on the NPT's Article IV, which guarantees non-weapons states the right to peaceful nuclear technology, including uranium enrichment equipment to produce fuel for nuclear power plants.

That same technology, with further enrichment, can produce material for nuclear bombs. Tehran denies that is the purpose of its long-secret uranium-enrichment program, but in his keynote address Annan said states like Iran "must not insist" on possessing such sensitive technology.

Following Annan to the U.N. podium, Mohamed ElBaradei, director-general of the International Atomic Energy Agency, renewed his call for a moratorium on new fuel-cycle facilities while international controls are negotiated.

ElBaradei has proposed putting nuclear fuel production under multilateral control by regional or international bodies. Rademaker reaffirmed President Bush's proposal for an outright ban on nuclear fuel technology, except in the United States and a dozen other countries that have it.

The Tehran government is negotiating on and off with Germany, France and Britain about shutting down its enrichment operations in return for economic incentives.

North Korea pulled out of the NPT in 2003 and said in February it has already built nuclear weapons. But the review conference is not expected to focus heavily on this first NPT defector, in order not to complicate efforts, via now-suspended six-party talks, to draw Pyongyang back into the treaty fold.