"The enrichment of uranium to 60 percent is underway at the Martyr Ahmadi Roshan nuclear facility" in Natanz, Ali Akbar Salehi of the Atomic Energy Organization of Iran said, quoted by Tasnim news agency.

"We are now getting nine grams (almost a third of an ounce) per hour," Salehi said later on state television.

Iran announced this week that it would boost its uranium enrichment to 60 percent purity in response to an attack Sunday on the Natanz nuclear facility that it blamed on arch-foe Israel.

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

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

Diplomatic negotiations aimed at ensuring the return of the United States to the JCPOA resumed this week in Vienna.

Britain, France and Germany -- the European parties to the JCPOA -- have expressed "grave concern" over Iran's latest enrichment move.

Iran's President Hassan Rouhani on Wednesday sought to allay Western concerns over the move, saying the Islamic republic's nuclear programme is "peaceful".