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.