U.N. inspectors visited a nuclear reactor Monday being built in central Iran, a facility that has been off-limits since April, state media reported.

"The team visited the 40-megawatt research reactor in Arak," the official Islamic Republic News Agency quoted an unnamed official as saying. "The inspection took some 5 hours."

The report did not provide additional details.

Experts from the U.N.'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.

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

Arak will produce plutonium once it is completed sometime in the next decade, and the U.N. 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 Iran of covertly trying to build a nuclear weapon.