The Iranian government says the country has resumed uranium enrichment to 20 percent at an underground facility, a level far above limits set by the 2015 nuclear deal with six major powers.
Government spokesman Ali Rabiei said on January 4 that President Hassan Rohani gave the order for the move at the Fordow facility -- the latest of several recent Iranian breaches of the international agreement that eased UN sanctions in exchange for curbs on Iran's disputed nuclear program.
But Tehran has gradually reduced its compliance with the accord since the United States unilaterally withdrew from the deal in 2018 and started imposing crippling sanctions on Iran.
Enriched uranium can be used to make reactor fuel but also nuclear warheads, with 90 percent purity considered weapons-grade.
There has been no confirmation from the International Atomic Energy Agency (IAEA) about Tehran's latest announcement.
However, the UN's atomic watchdog said on January 1 it had been informed by Tehran that it planned to resume enrichment up to 20 percent at the Fordow site, which is buried inside a mountain.
Ali Akbar Salehi, the head of the civilian Atomic Energy Organization of Iran, later said that Iran planned to enrich uranium up to 20 percent purity at Fordow "as soon as possible."
The step was mentioned in a law passed by Iran's parliament last month in response to the killing of a top Iranian nuclear scientist, which Tehran has blamed on Israel.
Iran currently enriches its uranium stockpile up to around 4.5 percent, which is above the 3.67 percent cap imposed by the 2015 nuclear pact.
An increase to 20 percent would shorten Iran's break-out time to a potential nuclear weapon, if it were to make a political decision to pursue a bomb.
The Iran nuclear deal also prohibits Tehran from enrichment at the Fordow facility, buried deep in a mountain to protect against air strikes.
Tehran has always denied pursuing nuclear weapons, saying its nuclear program was strictly for civilian purposes.