In a letter made available Monday to The Associated Press, senior Iranian envoy Ali Asghar Soltanieh suggests the weekend of Jan. 15 and 16 for the tour and says that meetings "with high ranking officials" are envisaged.
While no reason was given for the timing of the offer, it comes just weeks before Iran and the six powers follow up on recent talks that ended with agreement on little else but to meet again.
The new round between Tehran, and the permanent U.N. Security Council members the U.S. Russia, China, Britain, France plus Germany, is tentatively set for Istanbul, Turkey in late January.
It is meant to explore whether there is common ground for more substantive talks on Iran's nuclear program, viewed by the U.S, and its allies as a cover for secret plans to make nuclear arms something Tehran denies.
Instead, the Islamic Republic insists its uranium enrichment and other programs are meant only for peaceful purposes to generate fuel for a future network of nuclear reactors.
Diplomats from delegations at the table with Iran during the December talks in Geneva said Tehran made no commitments to talking about U.N. Security Council demands that it freeze uranium enrichment which can turn out both fuel and fissile warhead material. And Iranian negotiators flatly ruled out discussing such demands at the Istanbul meeting.
International worries are great because Tehran developed its enrichment program clandestinely and because it refuses to cooperate with the U.N.'s International Atomic Energy Agency probe meant to follow up on suspicions that it experimented with components of a nuclear weapons program. Tehran denies such work.
The offer of a visit comes more than three years after six diplomats from developing nations accredited to the IAEA visited Iran's uranium ore conversion site at Isfahan, which turns raw uranium into the gas that is then fed into enriching centrifuges. Participating diplomats then told reporters they could not make an assessment of Iran's nuclear aims based on what they saw at that facility in central Iran.
But the new offer appeared more wide ranging, both as far as the nations or groups invited and sites to be visited.
Dated Dec. 27, the four paragraph letter offered no details beyond offering an all-expenses paid "visit to Iran's nuclear sites."
But a diplomat familiar with its contents said it was mailed to Russia, China, Egypt, the group of nonaligned nations at the IAEA, Cuba, Arab League members at the IAEA, and Hungary, which currently holds the rotating EU presidency.
The U.S., the greatest critic of Iran's nuclear strivings, was not among those invited. China, and to a lesser degree Russia, have acted to dilute originally harsh sanctions measures proposed by the U.S. and its Western allies on the Security Council, Britain and France, leading to compromise penalties enacted by the council that are milder than the West had originally hoped for.
A U.S. State Department spokesman said Iran's continued enrichment activities are in violation of U.N. Security Council resolutions and "demonstrate Iran's disregard for its international obligations regarding its nuclear program."
"Acts such as Iran's invitation to several countries to tour its facilities are not a substitute for Iran fulfilling its obligations to cooperate with the IAEA and will not divert attention away from the core issues regarding Iran's nuclear program," said State Department spokesman Mark Toner.
The outreach to Moscow and Beijing represented by Tehran's offer to visit appeared to be an attempt to exacerbate any differences between the Eastern and Western powers meeting the Iranians in Istanbul.
The diplomat, who is accredited to the IAEA, also told the AP that Bushehr and Natanz were the venues to be toured and that meetings were planned with acting Foreign Minister Ali Salehi, the head of Iran's atomic agency, and Saeed Jalili, Tehran's chief nuclear negotiator. He asked for anonymity because his information is privileged.
The envisaged January startup of the Bushehr power plant, a project completed with Russian help but beset by years of delays, will deliver Iran the central stated goal of its atomic work the generation of nuclear power.
The Bushehr plant itself is not among the West's main worries because safeguards are in place to ensure that the spent fuel will be returned to Russia and cannot be diverted to weapons making.
The underground uranium enrichment facility in the central city of Natanz is of much deeper international concern. The U.N. Security Council has imposed four sets of sanctions on Iran for its refusal to suspend enrichment.
Since its enrichment activities were revealed nine years ago, Iran has expanded its program to the point where it now runs close to 9,000 centrifuges at Natanz, in central Iran. But after initial rapid growth, Iranian enrichment capacity has stagnated in recent years. Tehran has taken hundreds of centrifuges off line over the past two years, prompting speculation of technical problems.
An IAEA report in November said agency inspectors found the enrichment program at Natanz temporarily halted during a recent visit and Salehi later appeared to confirm speculation that the Stuxnet computer worm was responsible for the disruption.
The diplomat said the nonaligned group and the Arab League two staunch supporters of Iran's right to what Tehran insists is its peaceful use of nuclear energy had already accepted the invitation.
Ehab Fahzy, Egypt's ambassador to Austria and its chief delegate to the IAEA, told the AP he was awaiting instructions from Cairo. Officials from the other invited nations or groups were not answering telephones after office hours, while Soltanieh declined to comment.