Iran had denied UN inspectors' access to its underground nuclear facilities at Natanz, Austria Press Agency (APA) reported Monday, citing officials of the International Atomic Energy Agency (IAEA).

The officials believed that Iran's unprecedented refusal to allow the inspectors' access to the nuclear site could lead to further frustration of the international efforts to solve Iran's nuclear issue in a diplomatic way.

Iran has installed 164 centrifuges at the pilot plant at Natanz. UN inspectors had previously been allowed to monitor the plant.

Iran was due to reply to the incentive package put forward by Russia, the United States, China, Britain, France and Germany for solving its nuclear issue on Tuesday.

The UN Security Council adopted a resolution on July 31, urging Tehran to "suspend all enrichment-related and reprocessing activities, including research and development" by Aug. 31 or face possible sanctions.

Source: Xinhua