A new Iranian uranium enrichment facility has been detected in an underground nuclear complex in Isfahan, Iran, according to the UN's nuclear watchdog.

IAEA boss Rafael Grossi said it was not clear how advanced the facility is, The Caspian Post reports, citing foreign media.

"It is underground, but we haven't visited it yet," ​said Mr Grossi, ‌who is ⁠in ​Washington for ​a ‌conference ⁠and to hold ⁠talks with Trump administration officials.

Iran currently has about 440kg of 60 per cent enriched uranium, the IAEA estimates. This could be brought to 90 per cent - weapons grade - if a successful sprint was accomplished.

That would be enough for about 10 nuclear weapons, though the timetable is hard to calculate, officials have stressed in the past.

Mr Grossi said IAEA inspectors had to cancel a visit, so do not know whether the new site is "simply an empty hall" or whether centrifuges have begun to be installed. "There are ​many questions that we will only elucidate ​when we ‌are able to go back," he said.