An updated design of Iran's Fordo plant showed that the effect of the change "would be to significantly increase the rate of production of uranium enriched up to 60 percent," the International Atomic Energy Agency (IAEA) report said.

The rate of production will jump to more than 34 kilogrammes of highly enriched uranium per month, compared to 4.7 kilogrammes previously, added the report to the IAEA's board of governors.

In its report, the IAEA called on Iran to implement inspections "as a matter of urgency".

Those will "enable the agency to provide timely and technically credible assurances that the facility is not being misused to produce uranium of an enrichment level higher than that declared by Iran, and that there is no diversion of declared nuclear material," it added.

The IAEA said in a report last month that Iran planned to install 6,000 new centrifuges at its sites in Fordo and Natanz.

Iran insists on its right to nuclear energy for peaceful purposes and has consistently denied any ambition of developing weapons capability.

But according to the IAEA, it is the only non-nuclear-weapon state enriching uranium to 60 percent purity.

A landmark 2015 nuclear deal between Tehran and major powers aimed to give Iran relief from crippling Western sanctions in exchange for limiting its nuclear programme to prevent it from developing weapons capability.

Tehran kept to the terms of the accord, but in 2018, during Donald Trump's first US presidency, Washington unilaterally pulled out of the agreement and slapped heavy sanctions back on Iran, leading to the Islamic republic to step up its nuclear programme.

Trump returns to office in January.

Iran, Britain, France and Germany last week agreed at a meeting in Geneva to continue diplomatic talks.

The meeting came amid surging tensions even before Trump's return to the White House.