International Atomic Energy Agency head Rafael Grossi said Iran still had a way to go before getting the bomb, but added: "They're not far from it, you have to acknowledge."

Grossi likened the development of a nuclear weapon to a jigsaw puzzle, adding that Iran "has the pieces and they could eventually put them together one day".

Grossi was due in Iran later Wednesday for talks with senior officials.

The UN watchdog was tasked with overseeing Iran's compliance with the 2015 nuclear deal that fell apart after Donald Trump withdrew from it during his first term as US president.