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 concrete 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 Tehran, one of the world's largest oil producers.
Amano 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 work, which Tehran says is entirely 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.
"Without Iran's full engagement, we will not be able to start the process to resolve all outstanding issues, including those concerning possible military dimensions to its nuclear program," Amano said, according to a copy of his speech.
Iran's envoy to the IAEA, Ali Asghar Soltanieh, told reporters that Tehran would "continue" to cooperate with the UN agency but that its national security must be taken into consideration.
He said discussions were under way this week about the possibility of having another meeting with the IAEA. The U. agency was not immediately available for comment.
Iran says its nuclear program is aimed at producing electricity, not making nuclear bombs.
Iran says Parchin allegations 'baseless'
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 dispute - would agree to such a move, diplomats say. Beijing and Moscow have criticized 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 program was aimed at developing weapons.
In contrast, the IAEA has voiced mounting concern that Iran may be conducting research and development relevant to the assembly of a nuclear warhead.
"Iran is not providing the necessary cooperation to enable us to ... conclude that all nuclear material in Iran is in peaceful activities," Amano told the IAEA board.
He 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 citing satellite images as evidence that Iran has for several months carried out apparent "sanitization" work at Parchin to remove any evidence of illicit activity.
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.
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