ISTANBUL — Iran on Monday resumed 20 percent uranium enrichment at its sensitive Fordow nuclear facility, state media reported — a major step away from a 2015 nuclear deal struck with world powers.

The move could complicate the incoming Biden administration's plans to restart nuclear talks with Tehran. Iranian forces also seized a South Korean-flagged ship transiting the Persian Gulf.

The enrichment process at Fordow — an underground facility near the holy city of Qom — began earlier Monday, government spokesman Ali Rabie said. He said Iran had notified the International Atomic Energy Agency, the United Nations’ nuclear watchdog.

A spokesman for the agency said Monday that its inspectors were monitoring activities at Fordow and that IAEA Director General Rafael Mariano Grossi was expected to submit a report to the watchdog’s member states later in the day, Reuters reported.

Iran resumed 20% uranium enrichment officials said Jan. 4, defying a 2015 nuclear pact and complicating efforts by president-elect Biden to rejoin the deal. (Reuters)

Under the 2015 nuclear agreement, which restricted the size and purity of Iran’s enriched-uranium stockpile, Tehran is banned from enriching uranium at the Fordow site or even bringing uranium there for 15 years from the start of the accord. The site was revealed as a covert enrichment facility by Britain, France and the United States in 2009.

The nuclear deal originally agreed between Iran and six world powers, including the United States, allows Iran to enrich uranium to a 3.67 percent concentration of uranium-235, a fissile isotope, at another site and to maintain a small stockpile of it to use as fuel for its nuclear power reactors. Uranium enriched to 20 percent U-235 is suitable for use in an old, U.S.-supplied research reactor in Tehran that began operating in 1967.

However, the 20 percent enrichment level is also a relatively short, technical step from the 90 percent needed for the fissile material in a nuclear weapon. Iran began ramping up its nuclear activities after the Trump administration withdrew in 2018 from the agreement, which curbed Tehran’s nuclear program in exchange for major sanctions relief. Trump then began reimposing crippling sanctions. In response, Iran said it would progressively abandon some elements of the nuclear deal, notably the limits on the purity and size of its enriched-uranium stockpile, although it has maintained its commitment in the deal that it will not build or acquire nuclear arms.

The IAEA said in a statement Monday that it has informed member states that Iran “began feeding uranium already enriched up to 4.1 percent U-235 into six centrifuge cascades at Fordow for further enrichment up to 20 percent.” It said that “IAEA inspectors were present at the site” for the start of the process.

Iranian Foreign Minister Mohammad Javad Zarif wrote on Twitter that Iran’s “remedial action” followed “years of non-compliance by several other [nuclear deal] participants.” In a possible signal to the incoming Biden administration, he added: “Our measures are fully reversible upon FULL compliance by ALL.”

Israel, which considers Iran its greatest security threat and maintains that Tehran is seeking nuclear weapons, immediately condemned the Iranian move. Prime Minister Benjamin Netanyahu tweeted that it “cannot be explained in any way other than the further realization of its intention to develop a military nuclear program.”

“Israel will not allow Iran to produce nuclear weapons,” he added. Netanyahu has reportedly pressured Trump to consider a military strike against Iran’s nuclear facilities as he prepares to leave office.

Senior aides to President-elect Joe Biden have said that his administration would rejoin the nuclear deal if Iran also returned to compliance under the agreement.

The push for diplomacy was challenged in November when one of Iran’s most prominent nuclear scientists was killed in an ambush outside Tehran, an attack Iranian officials have blamed on Israel. Mohsen Fakhrizadeh was crucial to Iran’s plans to build a nuclear weapon nearly two decades ago, a program it abandoned by 2003, according to U.S. intelligence.

In recent days, tensions between Iran and the United States have soared in the Persian Gulf region as Tehran marked the first anniversary of the death of its most prominent military commander, Maj. Gen. Qasem Soleimani, who was killed by a U.S. drone strike in Baghdad.

The United States has deployed military assets to the region, including B-52 bombers, in what American officials said was an attempt to "deter" any Iranian retaliatory attacks on U.S. interests, which officials believe would most likely occur in Iraq.

On Sunday, acting defense secretary Christopher C. Miller said in a statement that the aircraft carrier USS Nimitz would remain in the Middle East "due to the recent threats issued by Iranian leaders against President Trump and other U.S. government officials." Iran has denounced the deployments as provocative.

"New intelligence from Iraq indicate that Israeli agent-provocateurs are plotting attacks against Americans — putting an outgoing Trump in a bind with a fake casus belli," Zarif, the Iranian foreign minister, wrote Saturday on Twitter, using a term for an action that justifies a war.

On Monday, Iranian state television reported the seizure of a South Korean-flagged tanker by the Revolutionary Guard in the gulf for "causing environmental pollution."

Iranian naval forces have harassed and impounded foreign-flagged tankers in the Persian Gulf in recent years. Western powers blamed Iran for a string of explosions targeting tankers near the United Arab Emirates in 2019.

The Revolutionary Guard said Monday that the South Korean vessel — MT Hankuk Chemi — had departed from a port in Saudi Arabia and was transiting the gulf for the UAE port of Fujairah when it was seized for "repeated violations of marine environmental laws," Iran's Tasnim News Agency reported.

The report said the ship was carrying 7,200 tons of ethanol and is now docked at Iran's Bandar Abbas port.

Steve Hendrix in Jerusalem contributed to this report.