Strike Set Back Iran’s Nuclear Program by Only a Few Months, U.S. Report Says

A preliminary classified U.S. report says the American bombing of three nuclear sites in Iran set back the country’s nuclear program by only a few months, according to officials familiar with the findings.

The strikes sealed off the entrances to two of the facilities but did not collapse their underground buildings, the officials said the early findings concluded.

Before the attack, U.S. intelligence agencies had said that if Iran tried to rush to making a bomb, it would take about three months. After the U.S. bombing run and days of attacks by the Israeli Air Force, the report by the Defense Intelligence Agency estimated that the program had been delayed, but by less than six months.

The report also said that much of Iran’s stockpile of enriched uranium was moved before the strikes, which destroyed little of the nuclear material. Iran may have moved some of that to secret locations.

Some Israeli officials said they also believed that the Iranian government had maintained small covert enrichment facilities so it could continue its nuclear program in the event of an attack on the larger facilities.

Other officials noted that the report found that the three nuclear sites — Fordo, Natanz and Isfahan — had suffered moderate to severe damage, with the facility at Natanz damaged the most. It is not clear whether the Iranians will try to rebuild the programs.

Former officials said that if Iran tried to quickly develop a bomb, it would be a relatively small and crude device. A miniaturized warhead would be far more difficult to produce, and the extent of damage to that more advanced research is not clear.

Current and former military officials had cautioned before the strike that any effort to destroy the Fordo facility, which is buried more than 250 feet under a mountain, would probably require waves of airstrikes, with days or even weeks of pounding the same spots.

American warplanes did hit the same spots at least twice on Saturday. B-2s dropped 12 GBU-57 Massive Ordnance Penetrator bombs — often referred to as “bunker busters” — on Fordo, and six aboveground entry craters are now visible, according to Brian Carter, the Middle East portfolio manager at the American Enterprise Institute.

But many military bomb experts believed that more than one day of strikes would be needed to complete the job.

The initial damage assessment suggests that President Trump’s claim that Iran’s nuclear facilities were “obliterated” was overstated. Congress had been set to be briefed on the strike on Tuesday, and lawmakers were expected to ask about the findings, but the session was postponed. Senators are now set be briefed on Thursday, and House members on Friday.

Since the strikes, Mr. Trump has complained to advisers repeatedly about news reports that have questioned how much damage was done, said people with knowledge of the comments. He has also closely watched the public statements of other officials when they are asked about the damage to the nuclear facilities, they said.

In a statement on Tuesday, Defense Secretary Pete Hegseth reiterated Mr. Trump’s early assessment.

“Based on everything we have seen — and I’ve seen it all — our bombing campaign obliterated Iran’s ability to create nuclear weapons,” he said. “Our massive bombs hit exactly the right spot at each target and worked perfectly.”

Officials cautioned that the five-page classified report was only an initial assessment, and that others would follow as more information was collected and as Iran examined the three sites. One official said that the reports people in the administration had been shown were “mixed” but that more assessments were yet to be done.

But the Defense Intelligence Agency report indicates that the sites were not damaged as much as some administration officials had hoped, and that Iran retains control of almost all of its nuclear material, meaning if it decides to make a nuclear weapon it might still be able to do so relatively quickly.

Officials interviewed for this article spoke on the condition of anonymity because the findings of the report remain classified.

The White House took issue with the assessment. Karoline Leavitt, a White House spokeswoman, said its findings were “flat-out wrong.”

“The leaking of this alleged assessment is a clear attempt to demean President Trump, and discredit the brave fighter pilots who conducted a perfectly executed mission to obliterate Iran’s nuclear program,” she said in a statement. “Everyone knows what happens when you drop 14 30,000-pound bombs perfectly on their targets: total obliteration.”

Elements of the intelligence report were reported earlier by CNN.

The strikes badly damaged the electrical system at Fordo, officials said. It is not clear how long it will take Iran to gain access to the underground buildings, repair the electrical systems and reinstall equipment that was moved.

There is no question that the bombing campaign “badly, badly damaged” the three sites, Mr. Carter said.

But initial Israeli damage assessments have also raised questions about the effectiveness of the strikes. Israeli defense officials said they had also collected evidence that the underground facilities at Fordo were not destroyed.

Before the strike, the U.S. military gave officials a range of possibilities for how much the attack could set back the Iranian program. Those ranged from a few months on the low end to years on the higher end.

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Some officials cautioned that such estimates are imprecise, and that it is impossible to know how long Iran would exactly take to rebuild, if it chose to do so.

Despite claims of the sites’ obliteration by Mr. Trump and Mr. Hegseth, Gen. Dan Caine, the chairman of the Joint Chiefs of Staff, has been more careful in describing the attack’s effects.

“This operation was designed to severely degrade Iran’s nuclear weapons infrastructure,” General Caine said that at the Sunday news conference.

The final battle damage assessment for the military operation against Iran, General Caine said on Sunday, standing next to Mr. Hegseth, was still to come. He said the initial assessment showed that all three sites “sustained severe damage and destruction.”

General Caine added that it was “way too early” to assess how much of Iran’s nuclear program remained.

Gen. Joseph L. Votel, the former commander of Central Command, said in an interview, that he had “a lot of confidence in the weapons systems used.” But he added: “I’m not surprised that elements survived. That’s why you do battle damage assessments, because everything can go as planned but there are still other factors.”

At a Senate hearing on Tuesday, Democrats also struck a more cautionary note.

“We still await final battle damage assessments,” said Senator Jack Reed of Rhode Island, the senior Democrat on the Armed Services Committee.

Military officials had said that to do more significant damage to the underground sites, they would have to be hit with multiple strikes. But Mr. Trump announced he would stop the strikes after approving the first wave.

U.S. intelligence agencies had concluded before the strikes that Iran had not made the decision to make a nuclear weapon, but possessed enough enriched uranium that if it decided to make a bomb, it could do so relatively quickly.

While intelligence officials had predicted that a strike on Fordo or other nuclear facilities by the United States could prompt Iran to make a bomb, U.S. officials said they do not know yet if Iran would do so.

Representatives of the Defense Intelligence Agency did not respond to requests for comment.

David E. Sanger contributed reporting.