Iran turned away UN inspectors wanting to examine its underground nuclear site in an apparent violation of the Nonproliferation Treaty, diplomats said.
Demanding anonymity because of the confidentiality of the information, they said Iran's unprecedented refusal to allow access to the facility at Natanz could seriously hamper international attempts to ensure Tehran is not trying to make nuclear weapons.
The news came as Iran's supreme leader Ayatollah Ali Khamenei said that Tehran would continue to pursue nuclear technology.
Khamenei's declaration on state television came on the eve of Iran's self-imposed August 22 deadline to respond to a Western incentives package for it to roll back its nuclear programme.
The United Nations has given Tehran until the end of August to suspend uranium enrichment.
"The Islamic Republic of Iran has made its own decision and in the nuclear case, God willing, with patience and power, will continue its path," Khamenei said.
He accused the US of pressuring Iran despite Tehran's assertions that it was not seeking to develop nuclear weapons.