By Fredrik Dahl
VIENNA, Aug 23 (Reuters) - Iran allowed a senior U.N. nuclear inspector to visit a facility for developing advanced uranium enrichment machines during a rare tour of all of the country's main atomic sites, an Iranian envoy said on Tuesday.
Ali Asghar Soltanieh, Iran's ambassador to the International Atomic Energy Agency, said last week's visit to Iranian nuclear facilities by IAEA Deputy Director General Herman Nackaerts showed Tehran's "100 percent transparency and openness".
Nackaerts, head of the IAEA's nuclear safeguards department, spent five days in Iran on a visit that coincided with a new push by Russia to revive deadlocked diplomatic talks between Tehran and the major powers.
Western nations suspect Iran is trying to use its nuclear program to develop atomic weapons. The Islamic Republic has denied the charge, saying it wants to produce nuclear energy. Refined uranium can have both civilian and military uses.
The IAEA earlier this year said it had been trying to visit sites linked to the manufacture of machines used to refine uranium since early 2008, but Iran had ignored all requests.
The IAEA confirmed that Nackaerts had been in Iran but gave no details of his trip. His department is responsible for checking that nuclear material is not used for weapons.
Soltanieh said Nackaerts had also visited a heavy water production plant, which the IAEA has long wanted to inspect.
The tour also included stops at Iran's enrichment plant at Natanz and other sites, which the agency inspects regularly, he told Reuters.
The two sides agreed during the visit on how U.N. inspectors would monitor activities at an underground bunker near the holy city of Qom, a facility to which Iran is preparing to shift its higher-grade uranium enrichment from Natanz, Soltanieh said.
Western officials and analysts say that by producing 20 percent enriched material Iran has taken a significant step closer to the 90 percent threshold suitable for atom bombs. Iran says the material is for making fuel for a research reactor.
Relations between Iran and the IAEA became increasingly strained over the last year, with the U.N. body voicing growing concern over possible military links to Tehran's nuclear work and repeatedly urging it to address those issues.
"This (Nackaert's visit) was a very big step to show our spirit of cooperation and transparency with the agency," Soltanieh said in Vienna, adding he had accompanied him in Iran.
"We arranged for him to visit all nuclear facilities that he wanted to see ... This was an indication of political will, that we are cooperating."
He said the IAEA official also met with Iran's atomic energy organization head, Fereydoun Abbasi-Davani, to discuss how to deal with "questions of the agency", without elaborating.
The IAEA has been investigating Western intelligence reports indicating Iran has coordinated efforts to process uranium, test explosives at high altitude and revamp a ballistic missile cone so it could take a nuclear warhead.
Iran says the allegations are baseless.
Tehran's refusal to halt enrichment has led to four rounds of U.N. sanctions on the major oil producer, as well as tighter U.S. and European Union restrictions.
Since talks between global powers and Iran foundered in January, Russia has advocated a phased plan in which Tehran would address concerns that it may be seeking nuclear weapons, and be rewarded with an easing of sanctions.
Tehran has given no indication that it is ready to address the Western powers' main concern, its enrichment activities.
Iran has started to move some of its centrifuges to the Fordow underground bunker that would be less exposed to any strike by Israel or the United States. Both countries have said military action is a possible last resort to stop Iran getting the bomb.
(Additional reporting by Zahra Hosseinian and Robin Pomeroy; Editing by Andrew Heavens)