Iran said on Monday it has begun enriching uranium up to 20% at an underground nuclear facility, a short, technical step to weapons-grade levels of 90% amid escalating tensions with the U.S and Israel.
Iranian state television quoted spokesman Ali Rabiei as saying that President Hassan Rouhani has given the order for the move at the Fordo facility.
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 atomic deal. A resumption of 20% enrichment could see that brinksmanship return.
Israel, which under Prime Minister Benjamin Netanyahu said the Jewish state will not allow Iran to manufacture nuclear weapons.
"Iran's decision to continue violations of its commitments, raise the level of enrichment and advance industrial capacity for underground uranium enrichment can not be explained in any way other than the further realization of its intention to develop a military nuclear program," he said in a statement.
The move comes after President Donald Trump withdrew the U.S. unilaterally from Iran's nuclear deal with world powers in 2018. In the time since, there have been a series of escalating incidents between the two countries.
Iran's decision comes after its parliament passed a bill, later approved by a constitutional watchdog, aimed at hiking enrichment to pressure Europe into providing sanctions relief. It also serves as pressure ahead of the inauguration of President-elect Joe Biden, who has said he is willing to re-enter the nuclear deal.
Iran informed the International Atomic Energy Agency last week that it planned to take the step.
Shielded by the mountains, Fordo is ringed by anti-aircraft guns and other fortifications. It is about the size of a football field, large enough to house 3,000 centrifuges, but small and hardened enough to lead U.S. officials to suspect it had a military purpose when they exposed the site publicly in 2009.
The 2015 deal saw Iran agree to limit its enrichment in exchange for sanctions relief. The accord also called for Fordo to be turned into a research-and-development facility.
Under Iran's former hard-line President Mahmoud Ahmadinejad, Tehran began enrichment at the 20% level. Israel, which has its own undeclared nuclear weapons program, feared Tehran was building an atomic bomb.
After the discovery of Fordo, the U.S. worked on so-called "bunker buster" bombs designed to strike such facilities. As Israel threatened at one point to bomb Iranian nuclear sites like Fordo, U.S. officials reportedly showed them a video of a bunker-buster bomb destroying a mock-up of Fordo in America's southwestern desert.
Up to now, Iran had enriched uranium up to 4.5%, in violation of the accord's limit of 3.67%. Experts say Iran now has enough low-enriched uranium stockpiled for at least two nuclear weapons, if it chose to pursue them. Iran long has maintained its nuclear program is peaceful.