"The team visited the 40-megawatt research reactor in Arak," IRNA quoted an unnamed official as saying. "The inspection took some 5 hours." The report did not provide additional details.

Experts from the UN's nuclear watchdog have said access to the reactor is critical to their review of Tehran's overall nuclear activities.

Iran agreed to the inspection last week after a meeting between International Atomic Energy Agency Deputy Director-General Olli Heinonen and a senior Iranian envoy.

IAEA spokesman Peter Rickwood declined to comment on Monday's reported visit.

But an IAEA official, who demanded anonymity because he was not authorized to comment to the media, confirmed that agency experts had visited the site. He said they were given full access and encountered no obstacles from the Iranian side.

Iran has blocked access to the site since early April, and the agency's inspectors have not visited since January 29 after the UN Security Council imposed limited sanctions on Iran.

Arak will produce plutonium once it is completed sometime in the next decade, and the UN Security Council has demanded that Iran stop construction. Plutonium, like uranium, is a possible pathway to nuclear arms.

Iran has vowed to continue its disputed nuclear program, insisting it is peaceful and geared solely toward producing electricity. The United States and key Western allies accuse it of covertly trying to build a nuclear weapon.