Iran has opened the doors of two atomic facilities to foreign diplomats, but representatives of the leading world powers locked in a dispute over Tehran's nuclear intentions declined to attend or were not invited.
The two-day tour reportedly takes the diplomats to the heavy water installation in Arak and the main uranium-enrichment facility at Natanz, both in central Iran.
The European Union declined the invitation, saying that UN nuclear inspectors, not diplomats, "are the people who have to inspect the Iranian nuclear facilities."
Russia said earlier this week that while Iran's invitation "deserved attention," it could not replace inspections by the UN's nuclear watchdog agency.
The tour comes ahead of talks with six world powers at the end of next week in Turkey chaired by EU foreign policy chief Catherine Ashton.
Iran is under four sets of UN sanctions over its refusal to suspend uranium enrichment, which Western powers fear could help Iran create material for atomic weapons.
compiled from agency reports