The UN nuclear watchdog held fresh talks with Iran Friday in Vienna, where it was expected to push for access to a suspected testing site near Tehran.
The agency's chief inspector Herman Nackaerts and deputy director general Rafael Grossi were meeting Iran's envoy to the agency, Ali Asghar Soltanieh, to try to get greater access to Tehran's contested nuclear programme.
The International Atomic Energy Agency is especially interested in the Parchin military base near the Iranian capital, where it believes suspicious explosives testing was carried out before 2003 and possibly after that.
Western powers and Israel suspect Iran of trying to develop a bomb behind the veil of its civilian nuclear programme, a charge denied by Tehran, which says it is developing civilian atomic power.
The agency says it was denied access to the Parchin site during two visits to Iran in January and February.
In a report last month, the IAEA said new satellite imagery indicated "extensive activities" at the base, where it said for years there had been virtually none.
This included the razing of two small buildings and what looked like a water run-off, suggesting a clean-up, experts said.
On May 21, IAEA chief Yukiya Amano visited Iran after talks in Vienna.
Afterwards, he noted that 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.
Earlier this week however, he hinted that a deal might still be some way off.
"If we do not have access to the Parchin site or other people, information and sites, then... we cannot give assurance that all the activities in Iran have peaceful purposes," he added.
Soltanieh refused to comment on the talks Friday, when he arrived for the meeting at the United Nations headquarters in Vienna, though two days earlier he expressed optimism that a deal would be concluded.
During talks in Beijing Friday, China's President Hu Jintao urged his Iranian counterpart Mahmoud Ahmadinejad to cooperate with the IAEA, state Xinhua news agency reported.
Hu also called on Iran to be "flexible and pragmatic" ahead of talks with the so-called P5+1 world powers -- the United States, Russia, China, Britain, France and Germany -- in Moscow on June 18-19.
Talks between the P5+1 and Iran were revived in Istanbul in April and they met 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.
On Thursday, Russia's President Vladimir Putin insisted Moscow supported Tehran's atomic programme as long as it was "peaceful."
US Secretary of State Hillary Clinton meanwhile called on Iran to be ready to take "concrete steps" on its disputed programme ahead of the Moscow talks.
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.
Iran's regime also insisted Thursday that Western powers must recognise Tehran's "right" to uranium enrichment if talks in Moscow are to advance. It says the the country needs the process to produce medical isotopes.