On Monday, an Iranian government spokesman said that the country had started enriching uranium of up to 20% at its underground Fordo nuclear facility.
It would put Tehran's program a technical step away from weapons-grade levels,
"A few minutes ago, the process of producing 20% enriched uranium has started in Fordow enrichment complex," government spokesman Ali Rabeie told Iranian state media.
The news comes amid heightened tensions between Iran and the US in the waning days of President Donald Trump's administration.
The outgoing president unilaterally withdrew from the nuclear deal in 2018 and ordered Tehran's top atomic expert's killing last year.
"The process of gas injection to centrifuges started a few hours ago, and the first product of uranium hexafluoride (UF6) gas will be available in a few hours," Rabeie said.
"The process has started after taking measures like informing the UN nuclear watchdog."
Iran’s decision to begin enriching to 20% a decade ago nearly brought an Israeli strike targeting its nuclear facilities, tensions that only abated with the 2015 nuclear deal.