International Atomic Energy Agency director Rafael Grossi informed IAEA member states that Tehran was boosting such capacity at its Natanz enrichment plant.

The agency verified on Saturday that "Iran had configured a new operational mode for the production of UF6 enriched up to 60 percent U-235," Grossi said in a statement to AFP.

This involved using two centrifuge cascades compared to one previously, he added.

The move takes Iran closer to the 90 percent purity level needed for use in a nuclear weapon.

Tehran had started in mid-April to enrich uranium to 60 percent.

The Islamic republic has gradually rolled back its nuclear commitments since 2018, when then US president Donald Trump withdrew from a multilateral nuclear deal and began imposing sanctions.

The 2015 deal known formally as the Joint Comprehensive Plan of Action, or JCPOA, gave Iran relief from sanctions in return for curbs on its nuclear programme.

Under the accord, Iran had committed to keep enrichment to 3.67 percent, though it had stepped this up to 20 percent in January.

Trump's successor Joe Biden said he hopes to revive the nuclear deal and indirect talks in Vienna, through European intermediaries, started in the Austrian capital in April.

But the talks broke up on June 20 without any discernible progress.

The EU said earlier this month that Iran is ready to resume talks under new president Ebrahim Raisi, and meetings could take place in Vienna from early September.

Meanwhile the United States on Monday urged Iran to return to the negotiations, voicing alarm over Iranian production of uranium metal reported by the UN nuclear watchdog.

According to the report presented by Grossi, Iran has made 200 grammes of the metal, after having announced such a project in July for research purposes.

The State Department said Washington believed Tehran "has no credible need to produce uranium metal".

Under the 2015 deal, Iran promised not to produce the metal, which can be used to build a nuclear bomb.