VIENNA: The U.N. nuclear watchdog chief pressed Iran on Monday to grant his inspectors access "without further delay" to a 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, said "activities" which 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 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 told the 35-nation governing board of the IAEA, according to a copy of his speech.

"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.

Amano said "no concrete results" had been achieved in a series of meetings between senior IAEA officials and Iran since January aimed at allaying concerns of possible military dimensions to Tehran's nuclear program.

"This is frustrating ... We consider it essential for Iran to engage with us without further delay on the substance of our concerns," Amano said.

"Iran is not providing the necessary cooperation to enable us to ... conclude that all nuclear material in Iran is in peaceful activities," he said.