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%.
It comes amid escalating tensions with the US and marks the Middle Eastern country's largest breach of the 2015 nuclear deal it struck with several other countries.
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 agreement. A resumption of 20% enrichment could see that brinksmanship return.
President Donald Trump withdrew the US 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 parliament recently 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.
The International Atomic Energy Agency (IAEA) did not immediately confirm Iran's announcement but said last week that Tehran had informed the watchdog 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 US 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, which is shielded by mountains and ringed by anti-aircraft guns and other fortifications. 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.
Israel, which under Prime Minister Benjamin Netanyahu has continued to criticise Iran’s nuclear programme, offered no immediate comment Saturday.
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.
Why is 20% enrichment such a big deal?
Nuclear reactors generally use enriched uranium between 3-5% purity, which would have been within the limits stipulated in the deal.
Any concentration below 20% is also considered to be a low-enriched uranium (LEU), while anything higher is said to be a highly-enriched uranium (HEU).
Why is this important?
Purity would need to be around 80-90% to be considered weapons-grade - and while this may sound like a big jump from 20%, it isn't as much as you think.
The most laborious part of the enrichment process is actually between the 0-20% mark, which Iran says it is now undertaking.
This is due to there being a large number of undesirable atoms that must be isolated and removed.
Once this milestone is reached, however, the boost from 20% to weapons-grade actually becomes much faster and much easier.