Vienna: The UN nuclear watchdog believes unspecified "activities" may be taking place at Iran's Parchin military facility that make its request to visit the site more urgent, Western diplomats said on Wednesday.
It was unclear whether the International Atomic Energy Agency (IAEA) thought Iran might be trying to clean the site or conceal something ahead of a possible visit by inspectors.
The Vienna-based UN agency has asked for access to Parchin as part of its probe into suspicions Iran may be seeking nuclear weapons capability, but Tehran has not granted it.
A senior Iran official said it was up to the military to decide if UN inspectors would be allowed to visit Parchin.
Diplomats who attended a closed-door briefing on Iran's nuclear programme by senior IAEA officials including chief inspector Herman Nackaerts on Wednesday said the UN agency was monitoring the site southeast of Tehran via satellite images.
One Western envoy said "some of the reports we have heard about possible sanitation" of the Parchin facility were "very concerning".
"It is very clear that Iran doesn't want the agency to go to Parchin because it has something to hide," he said.
The IAEA asked to visit the military complex after issuing a report in November that suggested Iran was pursuing military nuclear technology, an allegation Tehran denies.
The report helped trigger the latest round of US and European sanctions on Iran and bolstered hawkish politicians in Israel and the United States calling for pre-emptive military strikes on Iran's military sites.
IRAN "NOT RULING OUT" ACCESS
Despite talks with Nackaerts's team in January and February, Iran has not agreed to the request to access Parchin.
The report said the IAEA had information that Iran had built a large containment chamber there to conduct high-explosives tests which, it said, were "strong indicators of possible weapon development."
Another diplomat present at the briefing in Vienna quoted Nackaerts as saying there "may be some ongoing activities at Parchin which add urgency to why we want to go".
Nackaerts was reportedly asked by one of the diplomats present whether Iran might be trying to clean the Parchin site.
The IAEA was not immediately available for comment.
Iran's envoy to the UN agency earlier said his country had not refused access to Parchin during the talks with the IAEA.
"We are not ruling out access," Ali Asghar Soltanieh told reporters, suggesting it may happen once a broader agreement had been reached on how to address the IAEA's questions.
Soltanieh also dismissed statements by Western diplomats that the negotiations with the IAEA had been a failure, insisting that "substantial progress" was made and that the discussions should continue.
In Iran, a senior official said it was up to the military to decide if UN inspectors would be allowed to visit Parchin.
The official IRNA news agency quoted Fereydoun Abbasi-Davani, head of Iran's Atomic Energy Organisation, as saying that IAEA inspectors could in theory visit Parchin "whenever they want".
But it then quoted him as saying: "Whether or not IAEA inspectors can visit Parchin will be a decision for the country's military officials".
He said his own organisation was under no obligation "to show them anywhere they ask to visit in the country".
Suspicions about activities at the Parchin complex date back to at least 2004, when a prominent nuclear expert said satellite images showed it might be a site for research and testing relevant to nuclear weapons.
UN inspectors did in fact visit Parchin in 2005. But they did not see the place where the IAEA now believes the explosives chamber was built.