Iran has been preparing extra equipment for enriching uranium to higher levels, a confidential UN nuclear watchdog report obtained by Reuters said, a move which may increase tensions with the West over its atomic work.
The report said that Iran has now stockpiled over 2,400 kilograms of low-enriched uranium. That is twice the amount needed to make fissile nuclear warhead material.
Iran first started higher enrichment in February saying it wanted to produce fuel for a medical research reactor. But this raised suspicion abroad as it takes the enrichment level closer to the grade needed for atomic weapons.
Tehran says its nuclear program is for peaceful uses only but Western powers have been pushing for a fourth round of UN sanctions over the work.
Iran has now added a second set of centrifuges -- nuclear enrichment machines -- for refining uranium to 20 percent purity, but they are not yet operational, the International Atomic Energy Agency report said.
The Islamic Republic has told the agency that the extra machines will support the 20 percent enrichment work.
Tehran has however allowed the agency's inspectors better oversight at the site, something which they had been requesting for several months to ensure the material was not being diverted for military purposes.
The report also repeated that the IAEA remains concerned about possible current activity in Iran to develop a nuclear payload for a missile, and that Tehran must respond to the agency's queries as soon as possible.
Reuters and AP contributed to the report