Plans by Iran to install advanced centrifuges at its main nuclear enrichment plant in Natanz are "deeply worrying", France, Germany and the UK said on Monday.

The three governments, dubbed the E3, said the plans were contrary to a 2015 agreement between Tehran and world powers that aimed to restrain Iran's nuclear programme by barring sophisticated centrifuges.

The deal, officially called the Joint Comprehensive Plan of Action (JCPoA), is the best and currently the only way to monitor and constrain Iran's nuclear programme, the three countries said.

The UN's nuclear watchdog, the International Atomic Energy Agency (IAEA) reported last month that Iran had installed and begun operating advanced centrifuges at an underground section at Natanz.

"Iran's recent announcement to the IAEA that it intends to install an additional three cascades of advanced centrifuges at the Fuel Enrichment Plant in Natanz is contrary to the JCPoA and deeply worrying," the E3 said.

Under the terms of Iran's 2015 deal it is only meant to enrich uranium with a less sophisticated variety of centrifuges.

However, since May last year Iran has taken steps to violate that limit and several others laid down in the deal in retaliation for US President Donald Trump's withdrawal from the accord in 2018 and subsequent reimposition of sanctions.