Officials from the United Nations' nuclear watchdog were in Tehran yesterday seeking to restart an investigation into the "possible military dimensions" of Iran's nuclear programme.
Some of the eight-member delegation from the International Atomic Energy Agency (IAEA) arrived with metal briefcases, thought to contain equipment needed to inspect the Parchin military complex near the Iranian capital.
The agency suspects that until 2003, and possibly beyond, Iran carried out experiments there with explosives capable of triggering a nuclear weapon.
"We hope we will be allowed to go to Parchin," Herman Nackaerts, the IAEA's deputy director, said in Vienna as he boarded a plane to Iran for the second time in a month.
The UN agency also wants access to Iranian scientists and sensitive documents.
Iran, which insists its nuclear programme is peaceful, has expressed hope that a deal can be agreed but there was less optimism on the IAEA's part ahead of yesterday's intensive talks.
"Negotiating with Iran is quite a challenge," the agency's director general, Yukiya Amano, said last Friday.
"After making a step forward, there could be two or even three steps backward."
Iran, which has been under IAEA investigation for a decade, said on Tuesday it was ready to remove "any ambiguities" over its nuclear programme.
But its foreign ministry spokesman, Ramin Mehmanparast, appeared to link this to what he called recognition of his country's nuclear "rights" - meaning an acceptance by the West of Tehran's controversial uranium enrichment programme.
Iran also insisted Parchin was a conventional military site, not a nuclear one, and therefore was none of the IAEA's business.
Allegations about the facility were "ridiculous", Iran said, and based on "fabricated documents" provided by hostile spy agencies such as the CIA and Mossad - material Tehran complains it has not yet been allowed to see.
Mr Mehmanparast said on Tuesday that access to Parchin "could be discussed", but only in the context of a possible agreement.
Yesterday's talks were closely monitored by six world powers preparing for the resumption of wider, high-stakes nuclear negotiations with Iran in coming weeks, following November's re-election of the US president, Barack Obama.
The two sides have yet to agree on a date - although there are reports it would be at the end of this month - or a venue.
Dealings between Iran and the so-called P5+1 nations - the US, Britain, Russia, China, France and Germany - are separate from Tehran's nuclear negotiations with the IAEA, but they run in tandem.
The West's immediate aim is to persuade Iran to stop its enrichment of uranium to 20 per cent, which is a short technological step from producing bomb-grade material.
Iran has not ruled out making such a concession - provided it gets swift relief from choking, western-imposed sanctions in return.
Tehran also insisted it would never relinquish its "right" to enrich uranium up to 5 per cent, the level required to power civilian nuclear reactors.
Western officials, who have often accused Iran of stonewalling IAEA investigations, cautioned that even if Mr Nackaerts clinched a deal, they would remain sceptical until it was implemented by Tehran.
In a rare appearance before Iran's largely hostile parliament yesterday, Iran's president, Mahmoud Ahmadinejad - who once defiantly dismissed the sanctions as a "torn piece of paper" - acknowledged they were having a severe effect.
Naturally, he said, they were creating a series of problems, "including a slowdown in the country's growth, pressure on wide swaths of people who have a fixed income, disruption in foreign trade, and certainly a gap between classes".