The IAEA will “inform Iran of its intention to increase the frequency and intensity of its verification activities” at Fordo, the statement added.

At the other plant, in Natanz, Iran “continues to advance its enrichment activities” and now plans to install a second production building, the IAEA added.

Iran said Tuesday it had begun producing uranium enriched to 60 per cent at Fordo, an underground facility that reopened three years ago.

The move was part of Iran’s response to the UN nuclear watchdog’s adoption last week of a censure motion drafted by Western governments accusing it of non-cooperation.

It comes as talks to revive a 2015 landmark deal—which curbed Iran’s nuclear programme in exchange for sanctions relief—have stalled.

The deal started to unravel in 2018 when the US withdrew and reimposed sanctions. In response, Iran started to step up its nuclear programme.

An atomic bomb requires uranium enriched to 90 per cent, and 60 per cent is a significant step towards weapons-grade enrichment.

Iran has always denied any ambition to develop an atomic bomb, insisting its nuclear activities are for civilian purposes only.