The UN nuclear watchdog can push for access to Iranian military sites under the terms of a preliminary deal with Tehran, the head of the body told the Associated Press in an interview.
Iran reached a deal with six world powers on April 2 to allow UN inspectors to carry out more intrusive, short-notice inspections under an "additional protocol" - though there have been sharply differing interpretations from both sides on the details of that access.
"In many other countries from time to time we request access to military sites when we have the reason to, so why not Iran?" Yukiya Amano, the director general of the UN International Atomic Energy Agency (IAEA) was quoted as saying by AP.
"If we have a reason to request access, we will do so, and in principle Iran has to accept it," he added.
Iranian Supreme Leader Ayatollah Ali Khamenei, who has the final say for Iran on any deal, has ruled out any "extraordinary supervision measures" over nuclear activities and said military sites could not be inspected.
The Islamic republic dismisses accusations by Israel, Washington and other Western powers that it wants to develop nuclear bombs, saying its atomic research is for electricity generation and other peaceful purposes.
Negotiations are pushing ahead in Vienna this week as Iran and the six world powers - the United States, Russia, China, France, Britain and Germany - have set themselves an end-June deadline for a final deal to curb Iran's nuclear work in exchange for sanctions relief.
Meanwhile, Iran has been stalling a parallel IAEA investigation into its program which is in part aimed at clarifying whether its nuclear activities have ever had any possible military dimensions.
When asked whether the implementation of the Additional Protocol would help the IAEA's investigation, Amano said the agency did not know about that yet as it depended on the pace of Iran's cooperation.
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