Iran announced Monday that it had increased its uranium enrichment levels, bringing it closer to developing the capacity to produce a nuclear weapon within six months.

The resumption of enrichment to 20% was the latest in a series of escalations that have followed President Donald Trump’s decision to withdraw the United States from a 2015 nuclear agreement that had limited Iran to enrichment levels of 4% to 5%.

In another provocation, Iran seized a South Korean chemical tanker, citing “environmental and chemical pollution concerns,” the semiofficial Tasmin News Agency reported.

The seizure of the vessel, confirmed by the South Korean government, comes as Tehran is pressuring Seoul to release $7 billion in funds frozen because of U.S. sanctions.

Further adding to the tensions, the Pentagon said Sunday that it had ordered the aircraft carrier Nimitz to remain in the Middle East, just three days after directing the ship to head for home in an effort to de-escalate rising tensions with Tehran.

“Due to the recent threats issued by Iranian leaders against President Trump and other U.S. government officials, I have ordered the USS Nimitz to halt its routine redeployment,” the acting secretary of defense, Christopher C. Miller, said in a statement.

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A spokesman for the Iranian government, Ali Rabiei, told the state-run IRNA news agency Monday that President Hassan Rouhani had ordered the implementation of a law passed last week authorizing the new enrichment levels.

“A few minutes ago, the process of producing 20% enriched uranium has started in Fordow enrichment complex,” Rabiei told Iran’s semiofficial Mehr News Agency.

Fuel enriched to that level is not sufficient to produce a bomb, but it is close. Getting from current levels to 20% is far more difficult than going from that level to the 90% purity that is traditionally used for bomb-grade fuel.

Fordow is Iran’s newest nuclear facility and is embedded deep inside a mountain at a well-protected base of Iran’s Revolutionary Guard. Successfully striking it would require repeated attacks with the largest bunker-busting bomb in the U.S. arsenal.

The decision to bolster uranium enrichment, while not a surprise, was officially reached after the assassination in November of Iran’s top nuclear scientist, Mohsen Fakhrizadeh, long identified by U.S. and Israeli intelligence services as the guiding figure behind a covert effort to design an atomic warhead.

It also coincides with the first anniversary of the assassination of a revered military commander, Qasem Soleimani, in a U.S. missile strike.

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In a short statement, Prime Minister Benjamin Netanyahu of Israel accused Iran of continuing to act on its intention to “develop a military nuclear program.”

“Israel will not allow Iran to manufacture nuclear weapons,” Netanyahu said.

The European Union on Monday said that Iran’s decision to increase uranium enrichment would be “considerable departure” from commitments made in 2015.

Peter Stano, a spokesman for the bloc, said Brussels would wait until a briefing from the director of the United Nations’ International Atomic Energy Agency expected later Monday before deciding what action to take. France, Britain and Germany are all signatories to the 2015 accord.

The South Korean-flagged tanker was sailing in waters off Oman on Monday when Iranian authorities demanded that it move to Iranian waters for investigation. The ship had 20 crew members onboard, including five South Koreans.

“The Foreign Ministry and our Embassy in Iran have looked into the detailed circumstances of the seizure of our ship and confirmed the safety of the crewmen,” the South Korean Foreign Ministry said in a statement. “We are asking for the early release of the ship.”

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The Defense Ministry in Seoul said that it was dispatching the South Korean navy destroyer Choe Yeong to the waters where the tanker was seized, issuing precautionary warnings to other South Korean ships sailing in the waters. The navy destroyer has been on an anti-piracy mission in the region.

Iranian officials have always maintained that their nuclear ambitions are for peaceful purposes, not weapons. But they expressed fury and vowed revenge over the assassination of Fakhrizadeh, the nuclear scientist.

In December, Iranian lawmakers passed a law ordering an immediate ramping up of the uranium enrichment program and calling for the expulsion of international nuclear inspectors if U.S. sanctions were not lifted by early February, posing a direct challenge to President-elect Joe Biden.

Biden’s incoming national security adviser, Jake Sullivan, has expressed optimism that the 2015 nuclear accord could still be salvaged.

In a Foreign Affairs article published in May, Sullivan and Daniel Benaim, a Middle East adviser to Biden when he was vice president, argued that the United States should “immediately reestablish nuclear diplomacy with Iran and salvage what it can from the 2015 nuclear deal,” and then work with allies and Iran “to negotiate a follow-on agreement.”

Appearing on CNN on Sunday, Sullivan said that as soon as Iran reentered compliance with the 2015 nuclear deal there would be talks over its missile capabilities.

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“In that broader negotiation, we can ultimately secure limits on Iran’s ballistic missile technology,” Sullivan said, “and that is what we intend to try to pursue through diplomacy.”

But the missile program was not covered in the previous accord because the Iranians refused to commit to any limitations on their development or testing.

And that presupposes that the Iranians would be willing to return to the terms of the 2015 accord under any circumstances.