The move comes amid heightened tensions between Iran and the US in the waning days of the administration of President Donald Trump, who unilaterally withdrew America from Tehran’s nuclear deal in 2018.

That set in motion an escalating series of incidents capped by a US drone strike that killed a top Iranian general in Baghdad a year ago, an anniversary coming Sunday that has American officials now worried about possible retaliation by Iran.

Iran's decision to begin increasing its uranium quantity up to 20 per cent a decade ago nearly resulted in an Israeli strike targeting its nuclear facilities, tensions that only abated with the 2015 atomic deal. A resumption of 20 per cent enrichment could see that brinksmanship return.

Even Ali Akbar Salehi, the US-educated head of the civilian Atomic Energy Organization of Iran, offered a military analogy to describe his agency's readiness to take the next step.

“We are like soldiers and our fingers are on the triggers,” Salehi told Iranian state television. "The commander should order and we shoot. We are ready for this and will produce (20 per cent enriched uranium) as soon as possible.”

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 US President-elect Joe Biden, who has said he is willing to re-enter the nuclear deal.

The International Atomic Energy Agency (IAEA) acknowledged Iran had informed its inspectors of the decision by a letter after news leaked overnight Friday.

“Iran has informed the agency that in order to comply with a legal act recently passed by the countrys parliament, the Atomic Energy Organization of Iran intends to produce low-enriched uranium ... up to 20 percent at the Fordo Fuel Enrichment Plant,” the IAEA said in a statement.

The IAEA added Iran did not say when it planned to boost enrichment, though the agency “has inspectors present in Iran on a 24/7 basis and they have regular access to Fordo.” The parliamentary bill also called on Iran to expel those inspectors, though it appears Tehran still hasn't decided to take that step.

Salehi said Iran would need to switch out natural uranium in centrifuges at Fordo for material already enriched to 4 per cent to begin the process of going to 20 per cent. “It should be done under IAEA supervision,” Salehi added.

Since the deal’s collapse, Iran has resumed enrichment at Fordo, near the Shiite holy city of Qom, some 90 kilometres (55 miles) southwest of Tehran.

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 to be turned into a research-and-development facility.

Under Iran's former hard-line President Mahmoud Ahmadinejad, Tehran began 20 per cent enrichment. Israel, which has its own undeclared nuclear weapons program, feared Tehran was building a bomb.

After the discovery of Fordo, the US 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, US officials reportedly showed them a video of a bunker-buster bomb destroying a mock-up of Fordo in Americas southwestern desert.

Israel, which under Prime Minister Benjamin Netanyahu has continued to criticize Iran's nuclear program, offered no immediate comment Saturday.

As of now, Iran is enriching uranium up to 4.5 per cent, in violation of the accords limit of 3.67 per cent. Experts say Iran now has enough low-enriched uranium stockpiled for at least two nuclear weapons if it chooses to pursue them. Iran long has maintained its nuclear program is peaceful.

Iran separately has begun construction on a new site at Fordo, according to satellite photos obtained by The Associated Press in December.

Iran's announcement coincides with the anniversary of a US drone striking Revolutionary Guard Gen Qassem Soleimani in Baghdad last year. That attack later saw Iran retaliate by launching a ballistic missile strike injuring dozens of US troops in Iraq. Tehran also accidentally shot down a Ukrainian passenger jet that night, killing all 176 people on board.
As the anniversary approached, the U.S. has sent B-52 bombers flying over the region and sent a nuclear-powered submarine into the Persian Gulf.

On Thursday, sailors discovered a limpet mine on a tanker in the Persian Gulf off Iraq near the Iranian border as it prepared to transfer fuel to another tanker owned by a company traded on the New York Stock Exchange. No one has claimed responsibility for the mining, though it comes after a series of similar attacks in 2019 that the U.S. Navy blamed on Iran. Tehran denied being involved.

In November, an Iranian scientist who founded the country's military nuclear program two decades earlier was killed in an attack Tehran blames on Israel.