"We are producing about nine grams of 60% enriched uranium an hour," Ali Akbar Salehi, head of the Atomic Energy Organisation of Iran, said. "But we have to work on arrangements to drop it to 5 grams per hour."
Earlier, parliament speaker Mohammad Qalibaf said Iranian scientists had successfully started enriching 60 percent uranium at 12:40 a.m. local time (0810 GMT).
"The will of the Iranian nation makes miracles that thwart any conspiracy," Qalibaf said on Twitter.
Iran has said its decision to increase enrichment to its highest level ever was in response to sabotage at its key nuclear site at Natanz on Sunday by Israel.
Iran and global powers are meeting in Vienna to try to rescue a 2015 nuclear deal abandoned by Washington three years ago, in an effort potentially complicated by Tehran’s decision to ramp up uranium enrichment.
The 2015 agreement sought to make it harder for Iran to develop an atomic bomb - something it denies ever trying to do -- in return for lifting sanctions.
Abbas Araqchi, Iran’s chief nuclear negotiator at nuclear talks in Vienna, said earlier this week that Iran would activate 1,000 advanced centrifuge machines at Natanz.
An Iranian official told Reuters that "60% enrichment will be in small quantity" only.
Israel, which opposes the nuclear deal, has not formally commented on the incident at Natanz.