Iran began enriching uranium to 4.5%, breaking the limit set by its 2015 nuclear deal with world powers, the Islamic republic's media reported on Monday.
The acknowledgment by the spokesman of the Atomic Energy Organization of Iran comes a day after Iran pledged to break the deal's limit of 3.67%.
The decision to ramp up uranium enrichment came less than a week after Iran acknowledged breaking the 300-kilogram (660-pound) limit on its low-enriched uranium stockpile.
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Experts warn higher enrichment and a growing stockpile could begin to narrow the one-year window Iran would need to have enough material for an atomic bomb.
The future of the accord that President Donald Trump pulled the US from a year ago remains in question. While Iran's recent measures to increase enrichment and break its low-enriched uranium stockpile limit could be easily reversed, Europe has struggled to respond, even after getting a 60-day warning that the increase was coming.
Trump warned Tehran on Sunday that "Iran better be careful." He didn't elaborate on what actions the US might consider, but Trump told reporters: "Iran's doing a lot of bad things."
Iran has been closely monitored by inspectors from the International Atomic Energy Agency.
"We are aware of Iran's announcement related to its uranium enrichment level," the agency said Monday. "We are in the process of verifying this development."
Enriched uranium at the 3.67% level is enough for peaceful pursuits but is far below weapons-grade levels of 90%.
The Iranian news agencies ISNA and Fars reported the 4.5% enrichment figure, citing Behrouz Kamalvandi, a spokesman for Iran's nuclear agency.
Kamalvandi separately hinted in a state television interview aired on Monday that the country might consider going to 20% enrichment or higher as a third step if the material is needed.
That would worry nuclear nonproliferation experts, as 20% is a short technical step away from reaching weapons-grade levels of 90%. Kamalvandi also suggested using new or more centrifuges, which are limited by the deal.
Foreign Ministry spokesman Abbas Mousavi on Monday said Iran appreciated the efforts of some nations to save the deal but offered a jaded tone on whether Tehran trusted anyone in the negotiations.
