Iran has stepped up efforts to hide traces of possible nuclear weapons experiments at a key military site, the UN nuclear watchdog said in a new report on Thursday.
The International Atomic Energy Agency (IAEA) also said that the extensive clean-up activities conducted at the Parchin site "will have an adverse impact on the agency's ability to undertake effective verification."
The document, which was obtained by dpa, also said that Iran had been pushing ahead with efforts to enrich uranium to a level of 20 per cent, dpa reported.
At the underground Fordow facility, hundreds of new centrifuges for enriching uranium to this level have been installed since the IAEA's last report in May.
Western countries are concerned that the material being produced at Fordow and at a second site at Natanz could be easily turned into weapons material.
Iran denies having any plans to build nuclear weapons.
The IAEA on Thursday also said that it is displeased that Tehran had no been cooperating on investigating alleged nuclear weapons research and development projects. It has not granted access to the Parchin site, where nuclear warhead components were allegedly tested.