Ballistic experts say it's too soon to determine how much damage Iran's nuclear facilities sustained in the weekend attack.

On Saturday night, the United States targeted nuclear sites in Iran believed to be essential to the country's ability to produce nuclear weapons: a uranium enrichment site located in northwestern Iran -- deep inside a mountain in Fordo -- along with another uranium enrichment site in Natanz and the Isfahan Nuclear Technology Center, used for metallic uranium production.

Defense Secretary Pete Hegseth told reporters during an early morning briefing Sunday that the Iranian nuclear ambitions had been "obliterated," describing the military strike as "bold and brilliant."

President Donald Trump echoed that statement on Sunday night, writing on his social media platform, Truth Social, that "Monumental Damage" had been done to all of Iran's nuclear sites.

"Obliteration is an accurate term!" Trump wrote, adding that the "biggest damage" took place far below ground.

But other officials and experts have not gone as far when asked how much damage was done to the three sites.

Joint Chiefs Chairman Gen. Dan Caine said Sunday morning that it will "take some time" to assess how much damage has been done.

"Final battle damage will take some time, but initial battle damage assessments indicate that all three sites sustained extremely severe damage and destruction," Caine said.

There is currently a lack of understanding of the extent of the damage caused by the strikes, an Israeli official told ABC News on Sunday. In addition, it is unclear whether any material was moved from the Fordo site ahead of the strikes, the official said.

The plan was "well coordinated" and resulted from months of planning between the U.S. and Israel, the official said.

However, the aim of the overall operation was to degrade -- not destroy -- Iran's nuclear program, a separate Israeli source told ABC News.

"We did not say we eliminate the nuclear program," the source said. "We said remove the immediate existential threat, which is very different."

Other experts expressed skepticism of the damage that was done to Iran's nuclear sites.

A high-level European weapons systems and ballistics expert told ABC News that it seems "highly unlikely" that the bombs would be detonated in close vicinity to Fordo's underground structure, based on the calculated performance of the GBU-57, or the "bunker buster" bomb.

Calculations show that the GBU-57 cannot penetrate more than 65 feet through underground rock, but the Fordo enrichment site was built up to nearly 300 feet beneath the ground, within solid rock, the expert said.

"I personally can’t imagine that a lot of damage was done," especially if the underground facility was properly designed, the ballistics expert said.

The U.S. could have dropped several bombs to make the rock more fragile, allowing a subsequent strike to do more damage, the expert said.

The assessment of the degree of damage underground in Fordo is unclear, International Atomic Energy Agency Director-General Rafael Grossi told CNN's Fareed Zakaria on Sunday.

"Nobody else could be able to tell you how much it has been damaged," Grossi said.

The extent of the damage done to the above-ground facility in Natanz is very clear, Grossi said, adding that even the underground centrifuge -- used to enrich the uranium to higher grades -- suffered extensively due to the combined impacts that damaged the external power supply. Significant damage was also done at Isfahan, which has been sustaining attacks for several days, Grossi said.

The true extent of damage to the three sites -- including their nuclear infrastructure and what remains -- cannot be ascertained until inspectors from the IAEA or members of the international community are allowed to access them, Aditi Verma, an assistant professor at the University of Michigan's Department of Nuclear Engineering and Radiological Sciences, told ABC News.

It generally takes experts some time to complete assessments of damage from military action, so the full extent of the degradation to Iran's nuclear facilities will likely remain unknown for some time, John Erath, senior policy director of the Center for Arms Control and Non-Proliferation, told ABC News.

The environmental impacts from the strikes -- both chemical and radiological -- are likely to be confined to the immediate area, Verma said.

So far, there are no elevated levels of radioactivity being reported, Emily A. Caffrey, director for the Health Physics Program at the University of Alabama at Birmingham, told ABC News.

"That is not surprising considering the nature of the material that was presumed present at the site," Kathryn Ann Higley, distinguished professor of nuclear science and engineering at Oregon State University and president of the National Council on Radiation Protection and Measurements, told ABC News.

Uranium, even enriched uranium, is only slightly radioactive, Higley said. But the absence of radiological signature shouldn’t be taken as an indication of failure or success of the military action, Higley added.

But one "significant" concern is that the attacks will permanently estrange Iran from the international system of legal frameworks and norms that have prevented the Iranian government from building a weapon, Verma said.

"As we know, Iran has a history of developing clandestine enrichment facilities," Verma said.

Leaders from all sides should use the intervening time to look for a diplomatic solution that would make further military action unnecessary, Erath said.