The report, to be delivered as early as Thursday to a board meeting of the International Atomic Energy Agency, also said Tehran received sensitive technology that can be used as part of a weapons program earlier than it originally said it did.
The document said that while Iran had stated its plutonium separation experiments were conducted in 1993 "and that no plutonium had been separated since then," Iranian officials revealed two months ago that there had been linked experiments in 1995 and 1998.
The United States insists nearly two decades of clandestine activities revealed only three years ago indicate attempts by Iran to make weapons. Tehran has acknowledged purchasing much of its nuclear technology on the black market, but it insists its nuclear ambitions do not go beyond generating power.
Marked "highly confidential," the report to the U.N. nuclear monitor was made available by a diplomat accredited to the agency who demanded anonymity because he is not authorized to release such information to the media.
The three-page report took stock of the present stage of a two-year inquiry of Iran's nuclear activities. It suggested that some of the investigations were stalled, saying the IAEA "still needs to understand" the nature, dates and number of contacts between Iranian officials and nuclear black market intermediaries that supplied Tehran with much of its advanced technology - including centrifuges for uranium enrichment.
Asked about Tehran's nuclear program in an interview with BBC television, former Iranian President Hashemi Rafsanjani said it was "possible that, at times, Iran has not reported its activities."
"But from the time Iran decided to make such reports, it has made everything transparent," said Rafsanjani, who is a candidate in Friday's presidential election in Iran.
Rafsanjani said if he is elected president, he would make sure Iran lived up to all its obligations to the IAEA, but that he expected others to abide by the regulations as well.
The IAEA first revealed that Iran produced small amounts of plutonium as part of covert nuclear activities in November 2003, more than a year after revelations that Iran had run a secret atomic program led the agency to start investigating the country.
The agency has not linked the laboratory-scale experiments to weapons activity, nor has it said that any other parts of the program - including ambitious efforts to be able to enrich uranium - constituted evidence that Tehran has been trying to make weapons. But at the time, it criticized Tehran for not voluntarily revealing its plutonium work and other activities that could be linked to interest in making nuclear arms.
Plutonium can be used in nuclear weapons but it also has uses in peaceful programs to generate power.
Focusing on shipments of equipment for uranium enrichment - another technology that can be used in making weapons - the report said Tehran earlier this year provided documents showing that in at least two instances some components arrived in 1994 and 1995.
Those dates "deviate from information provided earlier by Iran," said the report, adding that one particular delivery had earlier been said to have reached the country in 1997.
Such discrepancies are important in agency investigations trying to establish how long Iran has been trying to assemble a program for enrichment, which can generate both fuel for power and weapons-grade uranium.
The report also outlined discrepancies about when Iranian officials said the first meetings with nuclear black marketeers were. It said clearing up inconsistencies about the shipments were essential to ensure "that there has been no other development or acquisition of enrichment design, technology or components by Iran.
While few other countries are as outspoken as Washington, dozens of nations - including some near Iran - are worried about Tehran's ultimate aims.
A confidential European Union briefing note made available to the AP cited the Saudi deputy foreign affairs minister, Prince Turki bin Mohammed bin Saud al-Kabira, as telling European envoys on the weekend that "Iran should cooperate for the safety of the whole region" in ensuring its nuclear aims were peaceful.
Much of Iran's nuclear program came from the network headed by Pakistani scientist A.Q. Khan, including the initial technology used in uranium enrichment.
Iran froze enrichment late last year as it started talks with France, Britain and Germany meant to reduce concerns about Tehran's nuclear ambitions.
The IAEA is pushing Iran to cooperate more with nuclear investigators, and agency chief Mohamed ElBaradei told board members Tuesday that more information was needed about Iran's uranium enrichment program. The report revealed Wednesday is to be delivered by one of his deputies, Pierre Goldschmidt.
The IAEA became concerned with Iran in 2003, when revelations of nearly two decades of secret nuclear activities surfaced. The work included uranium enrichment.
The meeting also will urge North Korea, the other key international proliferation concern, to return to six-nation talks meant to entice it away from making nuclear threats in exchange for economic and political concessions, the diplomats said.
Ahead of the meeting, South Korean President Roh Moo-hyun said Monday he was certain international nuclear talks with North Korea would resume and called for more flexibility in offering incentives to persuade Pyongyang to disarm.
Saudi Arabia is a relatively recent issue for the agency. The country has negotiated a now-outmoded deal with the IAEA that effectively excludes it from nuclear inspections in exchange for its pledge not to have anything worth inspecting.
After formal requests from the European Union, the United States and Australia to agree to an outside inquiry by agency inspectors, the Saudis will be under pressure to show some compromise at the meeting, said the diplomats.
In Riyadh on Sunday, the Saudi news agency cited an unidentified official as saying Saudi Arabia is willing to cooperate with the IAEA. But the Saudi official didn't mention inspection in his remarks.
By George Jahn