The UN's nuclear watchdog agency says that inspectors found some particles of uranium that had been enriched up to 27 percent at a site in Iran.
The International Atomic Energy Agency (IAEA) report noted that the enrichment was higher than the 20-percent enrichment declared by Tehran, but said it could have been the result of a technical error.
Such things "may happen for technical reasons beyond the operator's control," states the report, which was issued on May 25.
Twenty-percent enriched uranium is needed for nuclear fuel.
Uranium enriched to 90 percent is used for nuclear weapons.
Iran has stated that its nuclear program is for exclusively peaceful purposes, while the United States, Israel, and other countries suspect Tehran is seeking to build nuclear weapons.
Based on reporting by Reuters, AFP, and AP