Iran has formally dropped the limitations on uranium enrichment and the production of heavy water that were laid down in its landmark 2015 nuclear deal with world powers, an official from the Atomic Energy Organization of Iran announced on Wednesday.

The official said the move — which comes after the US backed out of the deal and reimposed many sanctions on Tehran — was in accordance with instructions from Iran’s Supreme National Security Council and was part of a recent 60-day Iranian ultimatum for renegotiating the pact, the regime-affiliated Iranian Students News Agency reported.

More details of the development were to be provided later in the day, the official, who asked to remain anonymous, said.

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On Tuesday night, Iran’s supreme leader claimed that enriching uranium to weapons-grade levels would not be a difficult task for his country — the latest threat from Tehran as tensions roiled the region amid the unraveling of the nuclear deal.

State-owned IRAN daily quoted Ayatollah Ali Khamenei as telling a group of officials during a meeting that “achieving 20 percent enrichment is the most difficult part. The next steps are easier than this step.”

Iran recently threatened to resume higher enrichment in 60 days if no new nuclear deal is in place, beyond the 3.67% permitted by the current deal. Weapons-grade uranium needs to be enriched to around 90%. However, once a country enriches uranium to around 20%, scientists say the time needed to reach 90% is halved. Iran has previously enriched to 20%.

Iranian officials, who maintain the nuclear program is for peaceful purposes, have said that they could reach 20% enrichment within four days.

Under the terms of the nuclear deal, Iran can keep a stockpile of no more than 300 kilograms (660 pounds) of low-enriched uranium. That’s compared to the 10,000 kilograms (22,046 pounds) of higher-enriched uranium it once had. It is also capped at storing 300 tons of heavy water, which it sells to Oman for use as a coolant in nuclear reactors.

On Monday, Iranian atomic agency spokesman Behrouz Kamalvandi said that his country was beginning to set in motion actions ordered last week by President Hassan Rouhani, who had warned Iran will breach enriched uranium and heavy water limits as a response to sanctions applied by Washington after last year pulling out of the deal.

Kamalvandi cited articles 26 and 36 of the deal, which discuss Iran’s right to suspend some of its commitments to the agreement if sanctions are brought back.

The United Nations’ nuclear watchdog, the International Atomic Energy Agency, says Iran has previously complied with the terms of the nuclear deal, which saw it limit its enrichment of uranium in exchange for the lifting of economic sanctions. But renewed American sanctions have wreaked havoc on Iran’s already-anemic economy, while promised help from European partners in the deal haven’t alleviated the pain.

Rouhani said in a televised address last Wednesday, a year after the US withdrew from the accord, that signatories to the deal have 60 days to come up with a plan to shield his country — already laboring under economic hardship — from the new sanctions imposed by US President Donald Trump. If negotiations fail, Iran will restart uranium enrichment to levels banned under the agreement, he warned.

Iran had stopped its sale of excess uranium and heavy water as a first step, Rouhani said, something required under the deal. A week earlier the US ended deals allowing Iran to exchange its enriched uranium for unrefined yellowcake uranium with Russia, and to sell its heavy water, which is used as a coolant in nuclear reactors, to Oman.

European powers have since rejected the “ultimatums” from Tehran, but vowed to fight to save the deal itself. The European Union has urged Iran to respect the international agreement, saying it aims to continue trading with the country despite US sanctions.

Following Rouhani’s announcement last week, Kamalvandi, the Iranian atomic agency spokesman, said that his country’s intention was to strengthen the deal and “bring it back on track.”

The Trump administration pulled America out of the deal in May 2018, saying it does nothing to stop Iran from developing missiles or destabilizing the Middle East. The Europeans insist that the pact is an important pillar of regional and global security and was never meant to address those other issues.

Sparking fears of war, already high tensions skyrocketed last week as US National Security Adviser John Bolton said that the United States was sending an aircraft carrier strike group and a bomber task force to the Middle East in a “clear and unmistakable” message to Iran.