A military statement said Israeli forces had "struck in order to obstruct access routes to the Fordo enrichment site" which US President Donald Trump said had been "totally obliterated" by the US strikes.

There has been speculation that Iran might have moved out some of its known 400-kilogramme stockpile of highly enriched uranium before the US bombing of its storage sites in the early hours of Sunday.

The International Atomic Energy Agency, which has been monitoring Iran's nuclear programme, called earlier Monday for access to the Islamic republic's nuclear sites to "account for" the uranium.

"There needs to be a cessation of hostilities for the necessary safety and security conditions to prevail so that Iran can let IAEA teams into the sites to assess the situation," the head of the UN's nuclear watchdog, Rafael Grossi, said.

Speaking to an emergency meeting of the organisation's board of governors in Vienna, he said that "at this time, no-one including the IAEA, is in a position to have fully assessed the underground damage at Fordo."

Asked about the location of Iran's enriched uranium stockpile on Sunday evening, Israeli Prime Minister Benjamin Netanyahu said Israel had "interesting intelligence" but declined to elaborate.

"We're following that," he told reporters. "We've been following it very closely... we have interesting intelligence on that which you'll excuse me if I don't share with you."

According to the IAEA, Iran had enriched uranium to 60 percent in 2021, a short step from the 90 percent required for use in a weapon.

Israel has maintained ambiguity about its own atomic arsenal, neither officially confirming nor denying it exists, but the Stockholm International Peace Research Institute has estimated it has 90 nuclear warheads.