United Nations ? Iranian President Mahmoud Ahmadinejad and Secretary of State Hillary Rodham Clinton engaged in a verbal nuclear exchange Monday on the U.N. stage, where nations gathered for a monthlong debate over the world’s ultimate weapons.

Speaking from the podium of the General Assembly Hall, Clinton accused Iran of “flouting the rules” of the Nuclear Nonproliferation Treaty with its suspect uranium enrichment program, and said it is “time for a strong international response.”

For his part, Ahmadinejad earlier rejected such allegations, saying Washington has offered not “a single credible proof.”

They were the opening salvos in four weeks of deliberations over how to improve the NPT, formally reviewed every five years in a meeting of all 189 treaty members — all the world’s nations except India, Pakistan, Israel and North Korea.