The confidential report, obtained by Reuters on Friday, listed several areas of questions the International Atomic Energy Agency (IAEA) has about Iran's atomic activities and called on Tehran to cooperate with its investigation.
It said the Vienna-based UN agency remained concerned about possible current activity in the Islamic republic to develop a nuclear payload for a missile, an allegation Tehran denies.
"Iran is not engaging with the agency in substance on issues concerning the allegation that Iran is developing a nuclear payload for its missile program," the report said.
The report is likely to add to Western suspicions that Iran is secretly bent on developing a nuclear weapons capability from its uranium enrichment program, and it may provide new arguments for tightening sanctions further on Tehran.
The report also said Iran had told the IAEA that it "would have to unload fuel assemblies" from the core of the Russian-built Bushehr nuclear reactor, which Iranian officials have earlier said would soon start generating electricity.
Iran did not give a reason for the move, which was announced a month after Russia said NATO should investigate a computer virus attack on Bushehr last year, saying the incident could have triggered a nuclear disaster on the scale of Chernobyl.
Despite a brief halt of enrichment work in November, Iran's total output of low-enriched uranium rose to reach a total of 3.61 tons, from 3.18 tons at the end of October, suggesting steady work despite technical woes and possible sabotage.
Experts say that amount could be enough for two bombs if refined much further. Iran denies Western accusations it is seeking to develop a nuclear weapons capability.
Enriched uranium can be used to fuel nuclear power plants, which is Iran's stated aim, or provide material for bombs if processed much further.
