Iran has turned away U.N. inspectors wanting to examine its underground nuclear site in an apparent violation of the Nonproliferation Treaty, diplomats and U.N. officials said Monday.

The officials, who spoke on condition of anonymity because of the confidentiality of the information, told The Associated Press that Iran's unprecedented refusal to allow access to the facility at Natanz could seriously hamper international efforts to ensure that Tehran is not trying to make nuclear weapons.