Amid rising nuclear tensions, more than 180 nations convened Monday to review the nonproliferation treaty, hearing calls from many sides for concessions by Iran and North Korea, America, Russia and others 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," Secretary-General Kofi Annan (search) said in opening the monthlong conference.

The U.N. chief urged nonweapons states like Iran (search) to renounce potential bomb technology, in return for international guarantees of nuclear fuel. But he also challenged Washington and Moscow to slash their nuclear arsenals irreversibly to just hundreds of warheads.

That call was echoed by a spokeswoman for a coalition of disarmament-minded nations. "We are greatly disappointed" by "unsatisfactory progress" toward disarmament by the big powers, said New Zealand's Marian Hobbs (search).

The U.S. representative rejected such criticism, pointing to recent arms-control agreements.

"We are proud to have played a leading role in reducing nuclear arsenals," said Stephen G. Rademaker, an assistant secretary of state.

Rademaker made clear the United States would seek, instead, to focus the conference on Iran and its nuclear-fuel program, and on North Korea.

Because of such differing priorities, treaty members were unable to agree on a complete agenda before the sessions began. Organizers hope to have agreement before the nuts-and-bolts work of committees begins next week.

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 political commitments are not legally binding, like a treaty, but give valuable support to nonproliferation initiatives. At the 2000 sessions, 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.

In his keynote address, Annan said all nations must work toward "a world of reduced nuclear threat and, ultimately, a world free of nuclear weapons."

The nuclear powers must find ways to rely less on nuclear deterrence, the U.N. chief said, and he called on Washington and Moscow "to commit themselves — irreversibly — to further cuts in their arsenals, so that warheads number in the hundreds, not the thousands."

Under the 2002 Moscow Treaty, the United States and Russia are to cut back their deployed warheads to between 1,700 and 2,200 each, by 2012.

When it's implemented, Rademaker said, "the United States will have reduced the number of strategic nuclear warheads it had deployed in 1990 by about 80%."

But the agreement has been criticized for not requiring destruction of excess warheads taken off deployment, or providing a transparent timetable and open verification of reductions.

The Iran question hinges on the NPT's Article IV, which guarantees nonweapons 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, and the United States alleges that's what Iran plans. "We dare not look the other way," Rademaker said.

Tehran denies the charge, but Annan said states such as Iran "must not insist" on possessing such sensitive technology, but instead should have access internationally to nuclear fuel.

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 proposes putting nuclear fuel production under multilateral control by regional or international bodies. Rademaker on Monday 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. Neither idea has generated widespread support.

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

Speaking for the European Union, Luxembourg's foreign minister, Jean Asselborn, cited its endorsement of international guarantees of access to nuclear fuel, on one hand, and at the same time said the EU "expects further reductions in the Russian and U.S. arsenals."

Malaysia's foreign minister, representing the 116-nation Non-Aligned Movement, said a "lack of balance" — the U.S. emphasis on nonproliferation over disarmament — "threatens to unravel the NPT regime."

"The nuclear weapons states continue to believe in the relevance of nuclear weapons," said Syed Hamid Albar. "We must all call for an end to this madness.

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