Iran's alleged studies into designing nuclear warheads remain a matter of serious concern and need "substantive explanations", the U.N. nuclear watchdog said on Monday.

The International Atomic Energy Agency (IAEA) also said in its latest quarterly report on Iran that Tehran had 3,500 centrifuges operational at its Natanz underground nuclear facility.

The agency said it had not been given access to Iranian nuclear-related sites that it asked to see in April.

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