The comments by Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), contrasted with a more upbeat assessment given by the Vienna-based U.N. agency after a meeting with Iranian officials last month.
The IAEA, whose mission it is to prevent the spread of nuclear weapons in the world, has been trying for a year to negotiate a so-called structured approach with Iran that would give it access to sites, officials and documents.
The IAEA's priority is to visit the Parchin military facility south-east of Tehran, where the agency suspects explosives tests relevant to nuclear weapons may have taken place, perhaps a decade ago. Tehran denies this.
