On the eve of the visit, Iranian envoy Ali Asghar Soltanieh said Friday that representatives of nonaligned nations, developing countries, the Arab League, Venezuela and Syria had accepted invitations to visit Iran's central Natanz enrichment facility and its still-unfinished heavy water reactor at Arak.
"This trip will offer the most transparency" regarding Iran's nuclear program, Soltanieh told The Associated Press, adding that the diplomats would be able to see "everything they wanted."
He declined to discuss which other nations had been invited or their responses. But China and the EU have in recent days publicly declined. And diplomats familiar with the issue said Russia, Switzerland, and Turkey and Brazil had also either turned down invitations or had not responded with less than a day to go before the departure from Vienna, where Soltanieh represents Iran at the International Atomic Energy Agency.
The diplomats all accredited to the IAEA, which is tasked with probing Iran's nuclear activities spoke on condition of anonymity because their information was privileged.
With crucial talks between Iran and six world powers in Istanbul just a week away, the timing of the nuclear tour and the choice of nations invited appeared a possible attempt to weaken unity among Iran's interlocutors.
Moscow and Beijing are part of the talks. At the same time, they are generally opposed to attempts by the other four the United States, Britain, France and Germany to sharpen U.N. sanctions on Iran over its refusal to stop activities that could be used to make nuclear weapons.
Neitherwere invited to Iran's weekend tour.
The United States and its allies fear that Iran is trying to make nuclear weapons, not nuclear energy, while Tehran asserts it is enriching uranium to make nuclear fuel and not weapons and says it will not negotiate over its right to enrich for peaceful uses.
Brazil and Turkey have recently emerged as important allies for Tehran in backing attempts to restart negotiations on a deal that would see Iran ship out some of its low-enriched uranium in exchange for fuel rods for a small reactor making medical isotopes.
Those talks stalled more than a year ago and the West considers them increasingly irrelevant as a way to slow Tehran's ability to make nuclear weapons by removing some material that could be enriched into weapons grade uranium.
While willing to talk about the deal at the Istanbul talks beginning next Thursday, the six powers want the discussions to focus on broader aspects of Iran's nuclear program, including its refusal to freeze enrichment despite four sets of U.N. sanctions.
Soltanieh denied the tour had been timed to sow division among the six powers, saying the visit and the Istanbul talks "had nothing to do" with each other.
"We simply created an opportunity ... to see our nuclear facilities but we respected their decision if they are not interested," he said.
In Tehran, acting Foreign Minister Ali Salehi, Iran's nuclear chief, said the invitations were intended as a trust-building measure, contending that outside of his nation no other country has put its nuclear facilities on display for others.
"All this is an indication of the peaceful nature of Iran's nuclear activities," he told the ISNA news agency.
The offer comes more than three years after six diplomats from developing nations 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 told reporters then they could not assess Iran's nuclear aims based on what they saw there.
The U.S. has mocked Iran's latest offer, calling it a "magical mystery tour" and saying it is no substitute for Iran fully cooperating with the IAEA the U.N. nuclear watchdog to prove that its nuclear program is strictly for peaceful purposes.
While Iranian officials deny that sanctions have hurt their nation's economy, the invitation may be a sign that Tehran is looking to ease the burden of the U.N. penalties.
Tehran's decision to return to talks could reflect some readiness to compromise on Security Council demands. Still, hopes are modest. The Istanbul meeting follows on a first round last month in Geneva that ended with little progress other than a decision to meet again.