Iran's apparent reluctance to let U.N. inspectors visit a military site near Tehran underlines the uphill task they face in getting the Islamic state to address suspicions it may be seeking to develop nuclear weapons, Western diplomats say.
They say the U.N. nuclear watchdog sought access to the Parchin complex during three days of talks in the Iranian capital, so far without any sign that Iran would agree to it.
More meetings are scheduled for later this month - rare direct dialogue in the long-running international dispute, which has deepened as the West pursues punitive embargos on Iranian oil and Tehran threatens retaliation.
The International Atomic Energy Agency (IAEA) named Parchin in a detailed report in November that lent independent weight to Western fears that Iran is working to develop an atomic bomb, an allegation Iranian officials reject.
The U.N. agency has not said whether the issue was among those it raised in the January 29-31 discussions in Tehran aimed at shedding light on possible nuclear-linked weapons development work, but diplomats accredited to the IAEA said it was.
The senior IAEA team requested access to Parchin, which Iran did not provide, one Western diplomat said.
He and others suggested that Iran had sidestepped the question rather than rejected it outright during the meetings with the IAEA delegation headed by the agency's global inspections chief, Deputy Director General Herman Nackaerts.
They asked to see a particular site and they never got an answer, another envoy said. The bottom line is: Iran did not engage the agency on the issues the agency wanted to discuss.
Iranian officials were not immediately available for comment. Foreign Minister Ali Akbar Salehi this week described the Tehran talks as very good, without giving details.
Last year's IAEA report laid bare a trove of intelligence pointing to research activities in Iran relevant for developing the means and technologies needed to assemble nuclear weapons, should it decide to do so.
One key finding was information that Iran had built a large containment chamber at Parchin southeast of Tehran in which to conduct high-explosives tests, which the U.N. agency said are strong indicators of possible weapon development.
Iran says its nuclear programme is entirely peaceful and rejects allegations of planned weapons as forged and baseless.
The IAEA said before its Tehran trip late last month that the overall objective was to resolve all outstanding substantive issues, referring to its growing concerns of possible military dimensions to Iran's nuclear programme.
But diplomats said the talks appeared to have made little concrete progress on the issues highlighted by the IAEA's report, which said Iran appeared to have worked on nuclear weapon design and secret research to that end may continue.
The IAEA delegation had also requested more information about a heavy-water reactor under construction and plans to build new uranium enrichment sites, one envoy said.
It is of great importance that the IAEA experts will have unfettered access to information, sites, equipment and people who have been involved in the military related activities, said Olli Heinonen, Nackaerts' predecessor who is now at Harvard University.
Tehran's history of hiding sensitive nuclear activity from the IAEA, continued restrictions on agency access and its refusal to suspend uranium enrichment - which can yield fuel for atom bombs - have drawn four rounds of U.N. sanctions.
The United States and the European Union seized on the IAEA report to ratchet up sanctions aimed at Iran's oil exports.
The outcome of the IAEA talks with Iran - a second round is planned for February 21-22 - is closely watched in Western capitals and Israel for signs of whether Iran's leadership may finally be prepared to give ground after a decade of pursuing its nuclear development goals or whether it remains as defiant as ever.
There was nothing achieved on this visit, the Western envoy said. I was never optimistic. This just reinforces my pessimism.
In a new sign of Iran's refusal to back down, Supreme Leader Ayatollah Ali Khamenei said Iran would retaliate over Western-backed oil sanctions and any threat of attack, insisting outside pressure would not halt its nuclear work.
Suspicions about activities at the Parchin military complex southeast of Tehran date back at least to 2004 when a prominent nuclear expert said satellite images showed it may be a site for research and testing relevant for nuclear weapons.
In 2005, U.N. inspectors visited the large site, but not the place where the IAEA now believes the explosives chamber was built.
(Editing by Michael Roddy and Andrew Heavens)