VIENNA - The UN nuclear watchdog chief on Monday urged Iran to open access of its military base Parchin to inspectors "without further delay."

Yukiya Amano, Director General of International Atomic Energy Agency (IAEA) also expressed "frustration", as no concrete results have been achieved "despite the intensified dialogue between the Agency and Iran since January 2012."

"We consider it essential for Iran to engage with us without further delay on the substance of our concerns," Amano said in his opening statement to the IAEA board of governors.

Though Iran stated in a letter in late August that the allegation of nuclear activities at Parchin was "baseless", "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," Amano added.

A recent IAEA report said that Iran has expanded its potential capacity to refine uranium in an underground site by doubling its centrifuges from 1,064 to 2,140. The report also said it has cleansed another site where the agency suspects the country conducted explosive experiments linked to production of nuclear weapons.

Iran's Foreign Minister Ali-Akbar Salehi responded that the recent report on Iran's Parchin Military Base lacks "technical significance," the semi-official ISNA news agency said.

Representatives from Iran and the five permanent members of the UN Security Council plus Germany, known as P5+1 have held their first meeting in 14 months in April in Istanbul, followed by meetings in Baghdad and Moscow.