The UN nuclear watchdog said in a confidential report that Iran had stepped up co-operation with the agency in recent months.
But the International Atomic Energy Agency (IAEA) said it has been unable to confirm Tehran is not enriching uranium.
"Additional work . . . is required to arrive at conclusions about Iran's statements that there have been no uranium enrichment activities in Iran involving nuclear material," said the conclusion of the report, obtained by the Reutersnews agency.
The IAEA also confirmed in the report it had found highly enriched uranium in environmental samples taken at a nuclear facility at Natanz, Iran.