The head of the UN nuclear watchdog pressed Iran on Monday to grant his inspectors immediate access to the Parchin military site, where they believe Tehran may have conducted explosives tests relevant to the development of nuclear weapons.
Yukiya Amano, director general of the International Atomic Energy Agency, also said it was "frustrating" that the IAEA and Iran had made no real progress in talks that began in January aimed at allaying concern about suspected atom bomb research.
Western powers may seize on his statement to a closed-door session of the IAEA's 35-nation governing board to strengthen their case for further international pressure on Iran, one of the world's largest oil producers.
Saying Tehran must address the substance of the UN agency's questions, Amano later told a news conference he was committed to intensifying the dialogue with the Islamic state but that no date had been set yet for a new meeting.
"We need to stop going around in circles discussing process ... Iran has the obligation to fully cooperate with us."
Asked about Iran's demand for access to documents which form the basis for the IAEA's suspicions about possible military dimensions to Iran's nuclear programme, Amano said he was ready to provide them "when appropriate".
He made his comments one day after Israeli Prime Minister Benjamin Netanyahu said that Israel and the United States were in discussions on setting a "red line" for Iran's nuclear programme, which Tehran says is peaceful.
Israel, believed to be the only nuclear-armed state in the Middle East, sees the possibility of Iran developing an atomic bomb as a threat to its existence and has said it may use military means if diplomacy and sanctions fail.
European Union heavyweights Britain, France and Germany called last week for new sanctions and Canada has unexpectedly severed ties with Iran.
IRAN SAYS PARCHIN ALLEGATION "BASELESS"
Iran's envoy to the IAEA, Ali Asghar Soltanieh, said Tehran would "continue" to cooperate with the U.N. agency but that its national security must be taken into consideration.
He told reporters that discussions were under way this week about the possibility of having another meeting with the IAEA.
Iran says its nuclear programme is aimed at producing electricity, not making nuclear bombs.
At the week-long IAEA board meeting, the United States and its Western allies want to isolate Iran further by adopting a resolution rebuking it for stonewalling the IAEA's investigation into its nuclear activities.
But it is unclear whether China and Russia - who are also part of a group of six world powers trying to find a diplomatic solution to the long-running nuclear dispute - would agree to such a move, diplomats say. Beijing and Moscow have criticised unilateral Western steps to punish Iran.
Russia last week starkly warned Israel and the United States against attacking Iran and said it saw no evidence that Tehran's nuclear programme was aimed at developing weapons.
In contrast, the IAEA has voiced mounting concern that Iran has been conducting research and development relevant to the assembly of a nuclear warhead.
Amano said "activities" that had taken place at the Parchin facility - a reference to suspected clean-up work there - would have an "adverse impact" on the IAEA's investigation, if and when it was allowed to go there. Iran has so far refused access.
Western diplomats cite satellite images as evidence that Iran has for several months carried out apparent "sanitisation" work at Parchin to remove any evidence of illicit activity.
Despite this, Amano said his inspectors had "powerful tools" at their disposal to discover any traces or other evidence.
Iran told the agency in a letter last month that the allegation of nuclear-linked work at Parchin, located southeast of the capital Tehran, was "baseless", Amano said.
"However, the activities observed further strengthen our assessment that it is necessary to have access to the location at Parchin without further delay in order to obtain the required clarifications," he said.