The UN nuclear watchdog held fresh talks with Iran on Friday in Vienna, amid calls that Tehran allow greater transparency of its nuclear programme, including access to a suspected testing site.
The meeting, between the agency's chief inspector Herman Nackaerts, deputy director general Rafael Grossi and Iran's envoy to the agency Ali Asghar Soltanieh, began at 10:00am (0800GMT) and was still ongoing in the late afternoon, with no details filtering out.
The International Atomic Energy Agency is especially interested in the Parchin military base near Tehran, where it believes suspicious explosives testing was carried out before 2003 and possibly after that.
Western powers and Israel suspect Iran is trying to produce a nuclear bomb, but Tehran insists it is merely developing atomic power for civilian purposes.
After a visit to Iran on May 21, IAEA chief Yukiya Amano said the two sides were close to a deal that would allow agency inspectors greater access to sites, people and documents tied to Iran's nuclear programme.
That would include access to the Parchin site, which the IAEA said it was denied during two visits to Iran in January and February.
In a report last month, it noted that new satellite imagery indicated "extensive activities" at the base, which "could hamper the agency's ability to undertake effective verification" of the site.
In other words, experts spoke of a clean-up, pointing to the razing of two small buildings and what looked like a water run-off.
This week, Amano again warned Iran that it needed to do more to alleviate international concerns over its nuclear drive.
"Iran is not providing the necessary cooperation to enable the agency to provide credible assurance about the absence of undeclared nuclear material and activities in Iran, and therefore to conclude that all nuclear material in Iran is in peaceful activities," he said.
The United States also said ahead of Friday's talks that it was not hopeful that a deal might be struck, while the European Union urged Iran to sign an accord with the IAEA.
Soltanieh, for his part, said he hoped the two sides would find a "common denominator" to reach a deal, adding he was optimistic.
The negotiations with the IAEA come ahead of a new round of talks between Iran and the so-called P5+1 world powers -- the United States, Russia, China, Britain, France and Germany -- in Moscow on June 18-19.
During a meeting in Beijing Friday, China's President Hu Jintao urged his Iranian counterpart Mahmoud Ahmadinejad to be "flexible and pragmatic" ahead of that meeting and to cooperate with the IAEA, Xinhua state news agency reported.
US Secretary of State Hillary Clinton meanwhile has said Iran should be ready to take "concrete steps" on its disputed programme ahead of Moscow.
Talks with the P5+1 were revived in Istanbul in April and were held again in Baghdad in May, although little was achieved.
A key source of dispute has been Iran's enrichment of uranium to 20-percent purity, bringing Tehran consistently closer to producing the 90-percent enriched uranium required to make a bomb, according to Western powers.
Russia's President Vladimir Putin said Thursday that Moscow supported Iran's atomic programme as long as it was "peaceful."
But Tehran insisted that Western powers must recognise its "right" to uranium enrichment -- which it says it needs to produce medical isotopes -- if talks in Moscow are to advance.
Barring progress there, an EU oil embargo against Iran will come into force on July 1, adding to a range of sanctions imposed under UN resolutions.
Iranian officials have repeatedly said in recent months that making, owning and using atomic weapons is "haram" (forbidden) under Islam.