The United States and Iran used the U.N. General Assembly's famous green marble podium Monday to trade punches over the Islamic republic's nuclear program, adding sudden drama to the normally staid opening of the international conference to review the Nuclear Non-Proliferation Treaty. President Mahmoud Ahmadinejad, the only head of state attending a meeting usually left to foreign ministers, defiantly sought to rebuff accusations that Iran was a nuclear outlaw by going on the offensive against the United States. The United States, represented by Secretary of State Hillary Rodham Clinton, accused Iran of trying to create a smokescreen to hide its own violations and said Tehran should not be allowed to try to undermine a broad international consensus to strengthen the 40-year-old treaty. The treaty is viewed as successful overall in dissuading countries from developing nuclear weapons but is seen as under threat as more nations express an interest in starting nuclear programs. The U.N. secretary general, Ban Ki-moon, said that expanding interest as well as new problems like possible nuclear terrorism made the treaty more important than ever. 'The nuclear threat remains real,' Ban said. 'It has evolved in new and varied forms.' In his speech, Ahmadinejad said that all nuclear powers tried to intimidate countries that had no nuclear weapons, but he called the United States the 'main suspect' in fostering a nuclear arms race. It has engendered worldwide hatred for being the first and only state to use a nuclear bomb, he said As Ahmadinejad spoke, members of delegations from a number of countries, including the United States and many European Union members, walked out of the General Assembly. The hall was about one-third full during his 35-minute address. The Iranian president said the United States and its allies had failed to provide a 'single credible proof' of allegations that Tehran was hiding an attempt to develop a nuclear bomb of its own. The Islamic republic had already accepted a compromise deal over enriching uranium for a research reactor, he said. In opening the conference, Ban urged Iran to prove that its nuclear program was solely for peaceful purposes and to accept a compromise deal offered to Tehran last fall. Iran is facing a fourth round of Security Council sanctions that are being negotiated separately. Under the terms of last fall's deal, Iran would send 1,200 kilograms, or more than 2,600 pounds, of enriched uranium to Russia and then to France for refinement into fuel rods for its research reactor in Tehran, which is used for medical purposes. Though it would not resolve the issue of a suspected military program, the deal was seen as a confidence-building measure. Western nations negotiating with Iran say that it has demanded, at different times, that the refinement take place on Iranian soil, or that the uranium be refined in smaller increments. Their interest in the deal is waning as Iran enriches increasing amounts of uranium.Its supply is now thought to be more than 2,100 kilograms, or about 4,600 pounds, enough for two bombs. Clinton noted that Tehran had never signaled to the International Atomic Energy Agency that it was ready to accept any proposal involving its research reactor. 'Iran has a history of making confusing, contradictory and inaccurate statements designed to convey the impression that it has adopted a flexible attitude toward the proposal,' she told a news conference. In her speech, Clinton accused Iran of jeopardizing the nonproliferation treaty by defying repeated calls from the Security Council and the atomic energy agency to be more transparent about its program. 'That is why it is facing increasing isolation and pressure from the international community,' she said. Canada's foreign minister, Lawrence Cannon, dismissed Ahmadinejad's speech as a 'publicity stunt.' He and other foreign ministers stressed that they did not want the issue of Iran to hijack the conference. Ban emphasized a few central issues that he said should be the focus of conference negotiations expected to last until the end of May: more nuclear weapons cuts, greater openness in national nuclear programs, an effort to get the three nations outside the treaty -- India, Pakistan and Israel -- to sign it and the establishment of a zone in the Middle East free of nuclear weapons. Clinton said the United States wanted to strengthen the measures facing any country that tried to withdraw from the treaty, as North Korea did in 2003. The country has since declared that it carried out two nuclear tests. The conference should consider automatic penalties for violating agreements on safeguards, Clinton said, like suspending international technical cooperation, and using financial and legal tools 'to disrupt illicit proliferation networks.' To bolster President Barack Obama's argument that the United States is fulfilling its obligations to reduce the American arsenal, the Pentagon on Monday declassified statistics showing that the United States now possesses 5,113 nuclear weapons, down 84 percent from its peak of 31,255 in 1967, at the height of the Cold War. (STORY CAN END HERE. OPTIONAL MATERIAL FOLLOWS.) The statistics' broad outlines have been known, in general terms, for many years. The Pentagon issued gross numbers on Monday, deliberately lumping together deployed weapons, those in storage and 'inactive'' warheads. The figures released for the current stockpile, a senior official said, do not include several thousand retired weapons awaiting complete dismantlement. American officials have been playing down prospects for any sweeping change emerging from the conference, suggesting that the greatest expected pressure on Tehran will be if a majority of countries endorse the treaty and there is some condemnation of Iran, even if it is rejected by a few 'outliers' and does not make it into the final document. Although Israel does not take part in the nonproliferation treaty, its arsenal, estimated at 100 to 200 warheads, has overshadowed the conferences in recent years. Egypt and other non-nuclear nations have refused to endorse stricter inspections and other global measures as long as Israel remains outside the treaty. A compromise worked out in 1995 called for a special conference on creating a nuclear-weapons-free Middle East, but it has never been convened. Egypt is again demanding that gathering. The U.S. position has been that such a treaty can be worked out only after a comprehensive Middle East peace plan. 