Iran intends to start enriching uranium to 20% purity, the UN's atomic watchdog says - its most significant breach of an international nuclear deal so far.

This remains short of the 90% required to make a nuclear bomb. But under the 2015 agreement Iran was supposed to keep enrichment below 4%.

Iran began breaching the deal after President Trump took the US out of it and re-imposed crippling sanctions.

However the UK, France, Germany, Russia and China all hope it can be revived.

The International Atomic Energy Agency (IAEA) said Iran had informed it of its plans to enrich to a purity of up to 20% at its Fordow Fuel Enrichment Plant, a facility built deep inside a mountain.

But the agency added: "Iran's letter to the agency ... did not say when this enrichment activity would take place."

Iran breached the 3.67% purity cap imposed by the nuclear deal in 2019 but the enrichment level had remained steady at up to 4.5% since then.

However the increase to 20% was mandated in a law passed by Iran's parliament last month in response to the assassination of the country's top nuclear scientist, Mohsen Fakhrizadeh.

The bill required the Iranian government to resume enriching uranium to 20% if sanctions on the country's oil and financial sectors were not eased in two months.

It also mandated the blocking of UN inspectors from Iran's nuclear facilities at Natanz and Fordow.