Iran said Tuesday that it would begin enriching uranium to a level of 60 percent purity, three times the current level and much closer to that needed to make a bomb, though American officials doubt the country has the ability to produce a weapon in the near future.
Deputy Foreign Minister Seyed Abbas Araghchi, Iran’s top nuclear negotiator, did not give a reason for the shift, but it appeared to be retaliation for an Israeli attack on Iran’s primary nuclear fuel production plant as well as a move to strengthen Iran’s hand in nuclear talks in Vienna.
The Israeli attack on Sunday diminishes Iran’s capacity to enrich uranium to 60 percent but it is unclear for how long.
Mr. Araghchi said that Iran had informed the International Atomic Energy Agency of its decision in a letter on Tuesday.
Iran also attacked an Israeli-owned cargo ship off the coast of the United Arab Emirates on Tuesday, officials said, the latest clash in its maritime shadow war with Israel. The attack was another sign of increased tensions in the region but was reported to have caused little to no damage.
The uranium enrichment announcement came as American intelligence agencies said that while Iran had gradually resumed production of nuclear material since President Donald J. Trump withdrew from the 2015 nuclear accord, there was no evidence it had resumed the work needed to fashion that material into a nuclear weapon.
“We continue to assess that Iran is not currently undertaking the key nuclear weapons-development activities that we judge would be necessary to produce a nuclear device,’’ the agencies said in their annual threat assessment report released on Tuesday.
The report said, however, that “if Tehran does not receive sanctions relief” — as Iran has demanded — “Iranian officials probably will consider options ranging from further enriching uranium up to 60 percent to designing and building a new” nuclear reactor that could, over the long term, produce bomb-grade material. That would take years.
The assessment would seem to give President Biden some breathing room as he enters negotiations in Vienna aimed at restoring some form of the nuclear agreement.
But there are still risks: Iran has a long relationship with North Korea, with which it has exchanged missile technology, and officials have for years been concerned that Iran might seek to buy proven nuclear-weapons technology from the North.
The White House press secretary, Jen Psaki, called Iran’s announcement on Tuesday “provocative,” and said it “calls into question Iran’s seriousness in regards to the nuclear talks.”
Mr. Araghchi, who was deeply involved in negotiating the 2015 nuclear accord between Tehran and the United States, also said Tuesday that Iran would replace the centrifuges damaged by the attack on Sunday on the nuclear plant at Natanz, where an explosion knocked the facility offline. He said that Iran would install an additional 1,000 centrifuges there to increase the plant’s capacity by 50 percent.
An Iranian official also provided a new estimate of the damage caused by the attack, saying that several thousand centrifuges were “completely destroyed.” That level of destruction takes out a large portion of Iran’s ability to enrich uranium.
But the full extent of the damage is unknown, and Iran presumably is vulnerable to continued attacks on its nuclear infrastructure. Until the electric power systems are rebuilt at Natanz, it would be impossible to make new centrifuges spin.
Iran is expected to replace the first-generation centrifuges damaged in the Israeli attack with more advanced, more efficient models.
Iran has another known production facility, Fordow, buried deep inside a mountain, but its capacity is limited.
Iran blamed Israel for the explosion at Natanz on Sunday, an assessment confirmed by American and Israeli intelligence officials. The Israeli government has not commented publicly.
Mr. Araghchi is in Vienna this week for indirect talks with the United States to revive the 2015 nuclear deal. The deal put restrictions on Iran’s nuclear program in exchange for the lifting of certain sanctions on Iran, and Mr. Biden has advocated restoring it in some manner.
After the United States withdrew from the deal and Mr. Trump imposed new sanctions, Iran abandoned its commitments under the agreement and increased its enrichment of uranium to 20 percent, a level that would have violated the terms of the accord.
Uranium enriched to 60 percent purity would be a further violation, and is a short step from bomb fuel, which is typically considered 90 percent or higher in purity. While uranium enriched to 60 percent can be used as fuel in civilian nuclear reactors, such applications have been discouraged globally because it can so easily be turned into bomb fuel.
Iran has been enriching uranium to roughly 20 percent purity at its Fordow plant, which uses about 1,000 centrifuges.
To raise the level to 60 percent purity, Iran would have to turn over roughly half of those machines onto the new enrichment job. Purifying it to 90 percent would require another hundred or so machines.
In an interview, Olli Heinonen, a former chief inspector for International Atomic Energy Agency, based in Vienna, said that theoretically Iran could go from 60 percent to 90 percent enrichment in a week, compared with a month or so starting from 20 percent.
“It’s not a huge difference,” he said.
“At this point, this is a demonstration,” Dr. Heinonen said of Iran achieving the 60 percent level. “They want to show that they can do it.”
The much more difficult step, he said, would be turning uranium enriched to 90 percent into the core of an atom bomb.
In another possible retaliation for the Israeli attack on Sunday, Iran attacked an Israeli-owned cargo ship, the Hyperion Ray, off the coast of the United Arab Emirates, on Tuesday.
According to a person familiar with the details of the ship’s voyage, the ship evaded the attack and was not hit. Israeli news media reported that it suffered light damage.
An Israeli security official said that Israel was seeking to reduce tensions in the Persian Gulf region and that it had no intention of responding with another attack on an Iranian vessel.
The Israeli Army, the Defense Ministry and the prime minister’s office all declined to comment.
In recent days, Israel had asked the United States for help protecting the ship, an American official said.
Israeli officials were concerned that it could be targeted by Iran’s Islamic Revolutionary Guards Corps in response to last week’s apparent mine attack by Israel on an Iranian military vessel in the Red Sea, the American official said.
A cargo ship owned by the same company, the Helios Ray, was attacked by Iran earlier this year.
Iranian officials also revealed more details about the Natanz attack on Tuesday, suggesting that the damage was greater than Iran previously reported.
Alireza Zakani, a member of Parliament and head of its research center, said on state television that “several thousand of our centrifuges have been completely destroyed,” representing a large portion of the country’s ability to enrich uranium.
He described official statements on Monday that the facility would be quickly repaired as false promises.
Foreign intelligence officials have said it could take many months for Iran to undo the damage.
Iranian officials have been livid about the security lapses that have allowed a series of attacks on Iran’s nuclear program over the past year, ranging from sabotage of nuclear facilities to the theft of classified documents to the assassination of Iran’s chief nuclear scientist. Most of these attacks were presumed to have been carried out by Israel.
Mr. Zakani criticized Iran’s security apparatus as lax, saying it had allowed spies to “roam free,” turning Iran into “a haven for spies.”
He said that in one incident, some nuclear equipment belonging to a major facility was sent abroad for repair and that when it returned the equipment was packed with 300 pounds of explosives. In another incident, he said, explosives were placed in a desk and smuggled inside the nuclear facility.
Iran has long maintained that its nuclear program is peaceful and aimed at energy development. Israel claims that Iran had and may still have an active nuclear weapons program and considers the possibility of a nuclear-armed Iran an existential threat.
The nuclear talks that began in Vienna last week have been delayed because a member of the European Union delegation tested positive for the coronavirus. The talks could resume as early as Thursday if the member tests negative.
Patrick Kingsley, Ronen Bergman and Steven Erlanger contributed reporting.