The exchange was contained in an 11-page report in which the agency painted a mixed picture of Iran's activities, and confirmed that Iran had begun to deploy a new generation of machinery to enrich uranium. The report, prepared by Mohamed ElBaradei, the director general of the agency, said Iranian officials had finally begun to answer a number of longstanding questions about its nuclear activities.
But officials with the United Nations agency said Iran had refused to deal with the evidence that served as the basis for American charges that Iran had tried to design a weapon. Much of it was contained in a laptop computer slipped out of the country by an Iranian technician four years ago and obtained by German and American intelligence agencies.
A National Intelligence Estimate published in early December by American intelligence agencies concluded, to the surprise of many in the White House, that Iran had suspended its work on a weapons design in late 2003, apparently in response to growing international pressure, adding that it was not clear whether the work had resumed.
That report threw into disarray the Bush administration's efforts to increase pressure on Iran. Since early last summer Mr. Bush has been trying to persuade the United Nations Security Council to ratchet up sanctions against Iran and pass a third resolution intended to cause more economic pain to the country.
But with the doubts now that Iran is actively pursuing a weapon, Russia and China -- which have deep commercial and oil ties to Iran -- have balked, agreeing only to a greatly watered-down set of sanctions that has yet to go to the Security Council for a vote.
Those sanctions are not based on suspected weapons work, but rather on Iran's continued refusal to halt enriching uranium. The new report confirms that Iran has begun deploying a new generation of centrifuges that can make fuel, for nuclear power plants or for weapons, much more efficiently.
"If this resolution is not voted, the credibility of the Security Council will be very much in doubt," R. Nicholas Burns, an under secretary of state, said on Friday. "The Security Council must now vote for new sanctions."
But Mr. Burns is leaving his post in a week, and inside the administration, it is becoming clear that the Iranians have been able to build centrifuges far faster than the administration could impose new sanctions.
Since the intelligence report came out, America's allies have spun all kinds of theories about the internal machinations that led to it, including that intelligence analysts were boxing Mr. Bush in, preventing him from taking military action against Iran's nuclear sites.
Officials who worked on the report have denied any such intent. The director of national intelligence, Mike McConnell, told Congress he now regretted how the intelligence estimate was presented, saying it failed to emphasize that Iran was moving ahead with the hardest part of any bomb project: producing the fuel. Designing a crude weapon is considered a far easier task.
With an eye to the decisions Mr. Bush's successor will have to make, two retired senior diplomats, William Luers and Thomas Pickering, and a research associate at the Massachusetts Institute of Technology, Jim Walsh, are publishing an article next week in The New York Review of Books urging the United States to use the intelligence report as a reason to open unconditional talks with Iran, and ultimately to establish an international fuel-production facility on its soil.
But Mr. Burns rejected that idea again Friday, and for years the administration has argued that if such a facility was allowed in Iran, its scientists would learn the difficult art of uranium enrichment and ultimately use that knowledge in a covert bomb project.
It was the evidence that Iran had, in the past, tried to design a weapon that is at the heart of the last confrontation between Iran and the nuclear agency.
Since 2005, the I.A.E.A. has urged the United States and other countries to allow it to show Iran the evidence obtained on the laptop, which intelligence officials have said once belonged to an Iranian technician with access to the country's nuclear program. But the United States. refused to allow the information to be shown to the Iranians until a few weeks ago.
Now that roadblock has been broken. The report says that a week ago the I.A.E.A was given permission to show original documents to the Iranians. In the report issued Friday, the agency described some of that evidence in public for the first time.
The most suspicious-looking document in the collection turned over to the I.A.E.A. was a schematic diagram showing what appeared to be the development of a warhead, with a layout of internal components. "This layout has been assessed by the agency as quite likely to be able to accommodate a nuclear device," the I.A.E.A. wrote. But that does not prove it was a nuclear warhead, and Iran argued that its missile program used "conventional warheads only."
David Albright, a former weapons inspector who now runs the Institute for Science and International Security, said: "The issue now is whether this is symptomatic of a comprehensive nuclear weapons effort, or just individual projects. Is it part of a plan to design and develop a weapon that can fit on a nuclear missile? And if so, why are so many pieces missing?"