JUDY WOODRUFF: Next: A country that may be developing nuclear weapons shows up at a conference to stop the spread of nuclear weapons.
Margaret Warner has our story.
PROTESTER: No nukes, no wars.
MARGARET WARNER: Holding signs reading "No Nukes, No War," thousands massed in New York City streets this weekend to call for a permanent end to nuclear weapons.
PROTESTER: We cannot continue to play with fire any longer.
MARGARET WARNER: They marched in advance of a United Nations conference convened to strengthen the current treaty to curb the spread of nuclear weapons.
U.N. Secretary General Ban Ki-Moon opened the conclave this morning.
BAN KI-MOON, United Nations secretary-general: The work you undertake this day is of immense importance to humankind.
MARGARET WARNER: Every five years, the signers of the 40-year-old Nuclear Non-Proliferation Treaty, or NPT, meet to review how well it's working and what else needs to be done. But looming over the month-long conference this time is an item not explicitly on the agenda: Iran's ongoing nuclear program.
And today made it clear this conference is going to be a showdown of sorts between the two main protagonists: Washington and Tehran.
LEONARD SPECTOR, Monterey Institute of International Studies: This is going to be a face-off between the United States and Iran.
MARGARET WARNER: Leonard Spector is deputy director of the non-proliferation center at the Nonproliferation Center at the Monterey Institute of International Studies.
LEONARD SPECTOR: The crucial issue on the front burner for everyone is dealing with the Iranian nuclear problem. It won't get played out that way. There will be other discussions. But underlying each one of these on nuclear energy, on disarmament, on building consensus, is, can we bring pressure to bear on Iran?
MARGARET WARNER: Iran is hard at work enriching uranium for civilian energy purposes, it insists. But the U.S. and its allies believe Tehran is bent on developing weapons and is pushing for tougher sanctions at the Security Council to try to thwart its ambitions.
Secretary-General Ban started off by calling on Iran to lay suspicions about its program to rest. But, at his turn to speak, Iranian President Mahmoud Ahmadinejad hit back, saying there was no credible proof his country was working on such weapons and insisting that it was the states with nuclear arms that were encouraging proliferation.
MAHMOUD AHMADINEJAD, Iranian president (through translator): There are reportedly more than 20,000 nuclear warheads worldwide, half of which belong to the United States. And the other competing groups continue to develop nuclear weapons under the pretext of deterrent. The trend constitutes a violation of obligations under the Nuclear Proliferation Treaty, or NPT.
MARGARET WARNER: Delegates from Britain, France, and the United States walked out of the hall as he spoke. But that didn't deter Ahmadinejad from also taking aim at the U.S. for retaining the right to use nuclear weapons.
MAHMOUD AHMADINEJAD (through translator): It is a misperception that it is OK to use nuclear weapons. Regrettably, the United States has not only used nuclear weapons, but also continues to threaten to use such weapons against other countries, including my country.
MARGARET WARNER: In a new nuclear strategy unveiled last month, the Obama administration pledged for the first time that the U.S. would never use nuclear weapons against a non-nuclear state, if, and only if, that state was abiding by its obligations under the NPT not to develop nuclear arms of its own.
Spector found Ahmadinejad's remarks entirely predictable.
LEONARD SPECTOR: Iran's strategy is to deflect attention away from Iran. This speech by Ahmadinejad was strictly about disarmament, and had nothing to do with compliance with the treaty or, you know, his own behavior.
MARGARET WARNER: And by disarmament, he's talking about the nuclear states, like the United States?
LEONARD SPECTOR: Principally, the United States.
MARGARET WARNER: Coming to the podium nearly four hours after Ahmadinejad, Secretary of State Hillary Clinton took direct aim at him.
HILLARY RODHAM CLINTON, U.S. secretary of state: This morning, Iran's president offered the same tired, false, and sometimes wild accusations against the United States and other parties at this conference. But that's not surprising.
Iran is the only country represented in this hall that has been found by the IAEA Board of Governors to be currently in noncompliance with its nuclear safeguard obligations, the only one. It has defied the U.N. Security Council and the IAEA and placed the future of the nonproliferation regime in jeopardy.
And that is why it is facing increasing isolation and pressure from the international community.
MARGARET WARNER: The secretary reminded her audience that U.S. and Russia recently agreed to reduce their arsenals. She also said the U.S. will, for the first time, disclose the size of its current nuclear weapons stockpile. That's 5,113 warheads, the Defense Department announced, down from 30,000 in the 1960s.
Now, Clinton told reporters later in the day, it's time for Iran to do its part.
HILLARY RODHAM CLINTON: Time and time again, I think we have demonstrated our commitment to the two-track process, the track of engagement and of moving forward together, and then the track of pressure.
Well, we are on the pressure track, but it is within the United Nations that we are seeking that pressure.
MARGARET WARNER: The U.S. hopes to end this conference with a tough statement on the importance of complying with the NPT, even though it's unlikely to win a unanimous vote.
LEONARD SPECTOR: If you can get many countries to support a statement like that, you have accomplished something and made it clear to Iran that it stands alone. Even if, in the end, they block it, they still have been isolated and have been taught a certain kind of lesson.
MARGARET WARNER: The conference concludes at the end of May, and a U.N. Security Council sanctions vote could follow soon after.