Ali Asghar Soltanieh, Iran's ambassador to the International Atomic Energy Agency (IAEA), suggested January 15-16 for the tour of the Natanz enrichment facility and heavy-water reactor in Arak, adding that meetings "with high-ranking officials" were envisaged.

It is not clear whether the envoys will travel to the Bushehr power plant, which was built under IAEA supervision last summer.

"The representatives of some European Union countries, NAM [Non-Aligned Movement], and some representatives of the five-plus-one [world powers] have been invited to visit our nuclear sites," Iranian Foreign Ministry spokesman Ramin Mehmanparast said earlier on Tuesday.

He said the invitation was part of Tehran's attempt to demonstrate "co-operation with the IAEA."

The United States does not appear to be among the invitees.

U.S. State Department spokesman Philip Crowley told the New York Times the offer is "a clever ploy."

"It's not a substitute for Iran's responsibilities to the IAEA," he said.

The last such trip was arranged by Iran in February 2007.

The tour will take place ahead of a fresh round of talks on Iran's controversial nuclear program with five permanent UN Security Council members (the United States, Russia, China, Britain and France) plus Germany, which is scheduled for late January in Istanbul, Turkey.

The West suspects Iran of developing nuclear weapons, but Tehran says its program is entirely peaceful.

The IAEA has expressed concern at lack of Iran's cooperation with its inspectors.