The U.S. and Israeli bombing of Iranian nuclear sites creates a conundrum for U.N. inspectors in Iran: how can you tell if enriched uranium stocks, some of them near weapons grade, were buried beneath the rubble or secretly hidden away?

Following last weekend’s attacks on three of Iran’s top nuclear sites – at Fordow, Natanz and Isfahan – President Donald Trump said the facilities had been “obliterated” by U.S. munitions, including bunker-busting bombs.

But the U.N. nuclear watchdog, the International Atomic Energy Agency, which monitors Tehran’s nuclear program, has said it’s unclear exactly what damage was sustained at Fordow, a plant buried deep inside a mountain that produced the bulk of Iran’s most highly enriched uranium.

IAEA chief Rafael Grossi said on Monday it was highly likely the sensitive centrifuges used to enrich uranium inside Fordow were badly damaged. It’s far less clear whether Iran’s 9 tonnes of enriched uranium – more than 400 kg of it enriched to close to weapons grade – were destroyed.

Western governments are scrambling to determine what’s become of it.

Reuters spoke to more than a dozen current and former officials involved in efforts to contain Iran’s nuclear program who said the bombing may have provided the perfect cover for Iran to make its uranium stockpiles disappear and any IAEA investigation would likely be lengthy and arduous.

Olli Heinonen, previously the IAEA’s top inspector from 2005 to 2010, said the search will probably involve complicated recovery of materials from damaged buildings as well as forensics and environmental sampling, which take a long time.

“There could be materials which are inaccessible, distributed under the rubble or lost during the bombing,” said Heinonen, who dealt extensively with Iran while at the IAEA and now works at the Stimson Center think-tank in Washington.

Iran’s more than 400 kg of uranium enriched to up to 60% purity – a short step from the roughly 90% of weapons grade – are enough, if enriched further, for nine nuclear weapons, according to an IAEA yardstick.

Even a fraction of that left unaccounted for would be a grave concern for Western powers that believe Iran is at least keeping the option of nuclear weapons open.

There are indications Iran may have moved some of its enriched uranium before it could be struck.

IAEA chief Grossi said Iran informed him on June 13, the day of Israel’s first attacks, that it was taking measures to protect its nuclear equipment and materials. While it did not elaborate, he said that suggests it was moved.

A Western diplomat involved in the dossier, who asked not to be identified because of the sensitivity of the issue, said most of the enriched uranium at Fordow would appear to have been moved days in advance of the attacks, “almost as if they knew it was coming”.

Some experts have said a line of vehicles including trucks visible on satellite imagery outside Fordow before it was hit suggests enriched uranium there was moved elsewhere, though U.S. Defense Secretary Pete Hegseth on Thursday said he was unaware of any intelligence suggesting Iran had moved it.

Trump has also dismissed such concerns. In an interview due to air on Sunday with Fox News Channel’s “Sunday Morning Futures”, he insisted the Iranians “didn’t move anything.”

“It’s very dangerous to do. It is very heavy – very, very heavy. It’s a very hard thing to do,” Trump said. “Plus we didn’t give much notice because they didn’t know we were coming until just, you know, then.”

The White House did not respond to a request for comment. The State Department referred Reuters to Trump’s public remarks.

A second Western diplomat said it would be a major challenge to verify the condition of the uranium stockpile, citing a long list of past disputes between the IAEA and Tehran, including Iran’s failure to credibly explain uranium traces found at undeclared sites.

“It’ll be a game of cat and mouse.”

Iran says it has fulfilled all its obligations towards the watchdog.

Before Israel launched its 12-day military campaign aimed at destroying Iran’s nuclear and missile capabilities, the IAEA had regular access to Iran’s enrichment sites and monitored what was inside them around the clock as part of the 191-nation Non-Proliferation Treaty aimed at preventing the spread of nuclear weapons, to which Iran is a party.

Now, rubble and ash blur the picture.

What’s more, Iran has threatened to stop working with the IAEA. Furious at the non-proliferation regime’s failure to protect it from strikes many countries see as unlawful, Iran’s parliament voted on Wednesday to suspend cooperation.

Tehran says a resolution this month passed by the IAEA’s 35-nation Board of Governors declaring Iran in breach of its non-proliferation obligations paved the way for Israel’s attacks, which began the next day, by providing an element of diplomatic cover. The IAEA denies that.

Iran has repeatedly denied that it has an active program to develop a nuclear bomb. And U.S. intelligence – dismissed by Trump before the airstrikes – had said there was no evidence Tehran was taking steps toward developing one.

However, experts say there is no reason for enriching uranium to 60% for a civilian nuclear program, which can run on less than 5% enrichment.

As a party to the NPT, Iran must account for its stock of enriched uranium. The IAEA then has to verify Iran’s account by means including inspections, but its powers are limited – it inspects Iran’s declared nuclear facilities but cannot carry out snap inspections at undeclared locations.

Iran has an unknown number of extra centrifuges stored at locations the U.N. nuclear watchdog is unaware of, the IAEA has said, with which it might be able to set up a new or secret enrichment site.

That makes hunting down the material that can be enriched further, particularly that closest to bomb grade, all the more important.

“Iran’s stockpile of 60% enriched uranium may not have been part of the ‘mission’ but it is a significant part of the proliferation risk – particularly if centrifuges are unaccounted for,” Kelsey Davenport of the Washington-based Arms Control Association said on X on Friday.

The IAEA can and does receive intelligence from member states, which include the United States and Israel, but says it takes nothing at face value and independently verifies tip-offs.

Having pummelled the sites housing the uranium, Israel and the U.S. are seen as the countries most likely to accuse Iran of hiding it or restarting enrichment, officials say.

Israeli Prime Minister Benjamin Netanyahu’s office did not respond to a request for comment for this story.

U.N. inspectors’ futile hunt for large caches of weapons of mass destruction in Iraq, which preceded the U.S.-led invasion in 2003, showed the enormous difficulty of verifying foreign powers’ assertions about hidden stockpiles of material when there is little tangible information to go on.

As in Iraq, inspectors could end up chasing shadows.

“If the Iranians come clean with the 400 kg of HEU (highly enriched uranium) then the problem is manageable, but if they don’t then nobody will ever be sure what happened to it,” a third Western diplomat said.

The IAEA, which answers to 180 member states, has said it cannot guarantee Iran’s nuclear development is entirely peaceful, but has no credible indications of a coordinated weapons program.

The U.S. this week backed the IAEA’s verification and monitoring work and urged Tehran to ensure its inspectors in the country are safe.

It is a long journey from there to accounting for every gram of enriched uranium, the IAEA’s standard.

The above-ground plant at Natanz, the smaller of the two facilities enriching uranium up to 60 percent, was flattened in the strikes, the IAEA said, suggesting a small portion of Iran’s enriched uranium stockpile may have been destroyed.

Fordow, Iran’s most deeply buried enrichment plant, which was producing the bulk of 60%-enriched uranium, was first seriously hit last weekend when the United States dropped its biggest conventional bombs on it. The damage to its underground halls is unclear.

An underground area in Isfahan where much of Iran’s most highly enriched uranium was stored was also bombed, causing damage to the tunnel entrances leading to it.

The agency has not been able to carry out inspections since Israel’s bombing campaign began, leaving the outside world with more questions than answers.

Grossi said on Wednesday the conditions at the bombed sites would make it difficult for IAEA inspectors to work there – suggesting it could take time. “There is rubble, there could be unexploded ordnance,” he said.

Heinonen, the former chief IAEA inspector, said it was vital the agency be transparent in real time about what its inspectors have been able to verify independently, including any uncertainties, and what remained unknown.

“Member states can then make their own risk assessments,” he said.