“They just punched through with these MOPs,” said David Albright, a former U.N. nuclear inspector who heads the Institute for Science and International Security, referring to the Massive Ordnance Penetrator bunker-busting bombs that the U.S. said it dropped. “I would expect that the facility is probably toast.”

But confirmation of the below-ground destruction could not be determined, noted Decker Eveleth, an associate researcher with the CNA Corporation who specializes in satellite imagery. The hall containing hundreds of centrifuges is "too deeply buried for us to evaluate the level of damage based on satellite imagery," he said.

To defend against attacks such as the one conducted by U.S. forces early on Sunday, Iran buried much of its nuclear program in fortified sites deep underground, including into the side of a mountain at Fordow.

Satellite images show six holes where the bunker-busting bombs appear to have penetrated the mountain, and then ground that looks disturbed and covered in dust.

The United States and Israel have said they intend to halt Tehran's nuclear program. But a failure to completely destroy its facilities and equipment could mean Iran could more easily restart the weapons program that U.S. intelligence and the U.N. International Atomic Energy Agency (IAEA) say it shuttered in 2003.

'UNUSUAL ACTIVITY'

Several experts also cautioned that Iran likely moved a stockpile of near weapons-grade highly enriched uranium out of Fordow before the strike early Sunday morning and could be hiding it and other nuclear components in locations unknown to Israel, the U.S. and U.N. nuclear inspectors.

They noted satellite imagery from Maxar Technologies showing "unusual activity" at Fordow on Thursday and Friday, with a long line of vehicles waiting outside an entrance of the facility. A senior Iranian source told Reuters on Sunday most of the near weapons-grade 60% highly enriched uranium had been moved to an undisclosed location before the U.S. attack.

"I don't think you can with great confidence do anything but set back their nuclear program by maybe a few years," said Jeffrey Lewis of the Middlebury Institute of International Studies at Monterey. "There's almost certainly facilities that we don't know about."

Arizona Sen. Mark Kelly, a Democrat and member of the Senate intelligence committee who said he had been reviewing intelligence every day, expressed the same concern.

"My big fear right now is that they take this entire program underground, not physically underground, but under the radar," he told NBC News. "Where we tried to stop it, there is a possibility that this could accelerate it."

Iran long has insisted that its nuclear program is for peaceful purposes.

But in response to Israel's attacks, Iran's parliament is threatening to withdraw from the Non-Proliferation Treaty, the cornerstone of the international system that went into force in 1970 to stop the spread of nuclear weapons, ending cooperation with the IAEA.

"The world is going to be in the dark about what Iran may be doing," said Daryl Kimball, executive director of the Arms Control Association advocacy group.

'DOUBLE TAP'

Reuters spoke to four experts who reviewed Maxar Technologies satellite imagery of Fordow showing six neatly spaced holes in two groups in the mountain ridge beneath which the hall containing the centrifuges is believed to be located.

General Dan Caine, chairman of the U.S. Joint Chiefs of Staff, told reporters that seven B-2 bombers dropped 14 GBU-57/B MOPs, 30,000-pound precision-guided bombs designed to drive up to 200 feet into hardened underground facilities like Fordow, according to a 2012 congressional report.

Caine said initial assessments indicated that the sites suffered extremely severe damage, but declined to speculate about whether any nuclear facilities remained intact.

Eveleth said the Maxar imagery of Fordow and Caine's comments indicated that the B-2s dropped an initial load of six MOPs on Fordow, followed by a "double tap" of six more in the exact same spots.

Operation Midnight Hammer also targeted Tehran’s main uranium enrichment facility at Natanz, he said, and struck in Isfahan, the location of the country's largest nuclear research center. There are other nuclear-related sites near the city.

Israel had already struck Natanz and the Isfahan Nuclear Research Center in its 10-day war with Iran.

Albright said in a post on X that Airbus Defence and Space satellite imagery showed that U.S. Tomahawk cruise missiles severely damaged a uranium facility at Isfahan and an impact hole above the underground enrichment halls at Natanz reportedly caused by a Massive Ordnance Penetrator bunker-busting bomb that "likely destroyed the facility."

Albright questioned the U.S. use of cruise missiles in Isfahan, saying that those weapons could not penetrate a tunnel complex near the main nuclear research center believed to be even deeper than Fordow. The IAEA said the tunnel entrances "were impacted."

He noted that Iran recently informed the IAEA that it planned to install a new uranium enrichment plant in Isfahan.

"There may be 2,000 to 3,000 more centrifuges that were slated to go into this new enrichment plant," he said. "Where are they?"