UNITED NATIONS — At a key U.N. disarmament conference yesterday, the United States lashed out at Iran and North Korea for their purported pursuit of atomic weapons and demanded that Iran dismantle its uranium-enrichment facilities.

But Iran said it had an "inalienable right" to develop nuclear technology for peaceful purposes and might restart its once-secret nuclear-energy program. The entrenched conflicts may set up the conference for failure, diplomats said.

At the opening of a monthlong review of the 1970 nuclear Non-Proliferation Treaty, U.S. negotiator Stephen Rademaker said the world needed to make sure countries such as North Korea and Iran could not exploit loopholes in the treaty to convert civilian energy programs into illegal weapons facilities.

"Some countries, such as Iran, are seeking these facilities, either secretly or with explanations that cannot withstand scrutiny. We dare not look the other way," he said.

Iran's threat two days ago to resume its nuclear-energy program and North Korea's launch of a missile test over the Sea of Japan last week added to the tensions surrounding this year's conference.

Rademaker repeated seven steps President Bush proposed last year to stop the spread of nuclear weapons through underground networks, including efforts to intercept the shipment of illegal weapons materials. The United States urged that nations such as Iran that don't comply with the treaty should be denied the technology guaranteed under it and that countries such as North Korea that withdrew from the treaty should be sanctioned.

The 35-year-old treaty requires 183 nations to renounce nuclear ambitions and for the five major nuclear powers — the United States, Russia, China, France and Britain — to eventually eliminate their nuclear arms. Under the 2002 Moscow Treaty, the United States and Russia are to reduce their warheads by two-thirds by 2012. Israel, India and Pakistan also have nuclear capability, but they have not signed the treaty.

U.N. Secretary-General Kofi Annan conceded that the treaty was flawed and needed strengthening. He urged the United States and Russia to cut their weapons arsenals and work toward "a world free of nuclear weapons." He supported Iran's right to have access to nuclear fuel but said Iran "must not insist" on developing sensitive technology at home.

The chief of the United Nations' nuclear-watchdog agency, Mohamed ElBaradei, asked all nations to stop processing nuclear material until new international controls could be negotiated. He has proposed putting nuclear-fuel production under the control of the United Nations or other multilateral bodies, a suggestion the United States is considering.

The conference occurs every five years to assess progress and set new goals under the treaty; the final agreement is not legally binding.

The Bush administration would like to focus on the problems posed by North Korea and Iran, while other countries want to push the nuclear powers to reduce their stockpiles more quickly.

The divergence of priorities meant that the conference began yesterday without an agreed-upon agenda, and diplomats worried that squabbling over procedure among the 189 attending countries would distract from solving crucial issues.