Melissa Fleming told German Deutschlandfunk radio that this estimate still leaves time for diplomatic efforts to counter the potential threat.
Fleming said it is incomprehensible that Tehran is not opening its uranium-enrichment program to international inspection.
She also said that Iran has a basic right to operate nuclear facilities, provided that this is done in a transparent and legal way.
Fleming's remarks came days after Iran announced that it is now capable of producing nuclear fuel on an industrial scale through enrichment.
Two sets of UN sanctions have been imposed on Iran over its refusal to halt such work.
"We have a schedule that's laid out in [UN Security Council] Resolution 1747 and yesterday's comments and statements by the Iranian president suggest we will be back at the Security Council dealing with this yet again," acting U.S. Ambassador to the UN Alejandro Wolff told journalists on April 10. "We have two resolutions dealing with this issue. Yesterday's rejection of the approach laid out in those resolutions I think makes clear that Iran continues to disregard its obligations."
Western nations suspect Iran's nuclear program is aimed at producing atomic weapons. Tehran denies this.
A control panel at the Bushehr nuclear power plant (Fars)
CASCADES AND CENTRIFUGES: Experts and pundits alike continue to debate the goals and status of Iran's nuclear program. It remains unclear whether the program is, as Tehran insists, a purely peaceful enegy project or, as the United States claims, part of an effort to acquire nuclear weapons.
On June 7, 2006, RFE/RL correspondent Charles Recknagel spoke with nuclear expert Shannon Kile of the Stockholm International Peace Research Institute in Sweden to help sort through some of the technical issues involved. "[Natanz] will be quite a large plant," Kile said. "There will be about 50,000 centrifuges and how much enriched uranium that can produce [is] hard to say because the efficiency of the centrifuges is not really known yet. But it would clearly be enough to be able to produce enough [highly-enriched uranium] for a nuclear weapon in fairly short order, if that's the route that they chose to go...." (more)