Donald Trump has said he does not care about Iran’s stock of highly enriched uranium (HEU), arguing it was deep underground and could be monitored by satellite, raising questions about one of the key US justifications for the war.

Experts said that if the US-Israeli offensive against Iran concluded with the Tehran government still in control of its 440kg HEU stockpile, it would be significantly closer to the capability of making nuclear warheads than if the US had pursued a potential negotiated settlement that was on the table at the time the US and Israel launched the war on 28 February.

Asked about the stockpile by Reuters news agency on Wednesday, Trump said: “That’s so far underground, I don’t care about that.”

“We’ll always be watching it by satellite,” he added.

In his address to the nation from the White House on Wednesday night, Trump elaborated: “If we see them make a move, even a move for it, we will hit them with missiles very hard again.”

Unless they were intended as a ruse to put Tehran off its guard, the president’s remarks appeared to rule out a risky military mission to retrieve the HEU stockpile, which Iran is believed to have hidden down deep underground shafts.

The apparent decision to leave the HEU, which is roughly enough for about a dozen warheads, in Iran appeared to conflict with Trump’s assertions that one of the principal war aims was to ensure it could never make a nuclear bomb.

He has repeatedly claimed, since starting the war, that Iran had been two to four weeks from making a nuclear weapon and firing it at the US and Israel, a claim rejected as absurd by most experts.

Nuclear proliferation experts say that if the HEU stock remains under Iranian control at the end of hostilities, it would leave Tehran significantly closer to the capability of making nuclear bombs than the proposed settlement being negotiated in Geneva on 26 February, two days before the war began.

In those US-Iran talks, Iranian officials have said they had proposed diluting the HEU stockpile to low-enriched uranium, and reportedly agreed to keep only a much smaller stock of enriched uranium on its territory.

The Iranian proposal would have also included a multiyear pause in any uranium enrichment and paved the way for a restoration of a comprehensive monitoring regime by the UN nuclear watchdog, the International Atomic Energy Agency (IAEA).

The Omani mediators at the Geneva negotiations thought that significant progress had been made, as did the UK’s national security adviser, Jonathan Powell, who was in Geneva at the time with British nuclear experts.

Another, more technical, round of talks was due to take place the following Monday in Vienna but it never happened, because the US and Israel launched their attack.

“We are actually less secure now from the nuclear threat than we were before he started the war, because they still have the material and we still have no greater insight into the material and what they might do with it,” said Emma Belcher, a nuclear expert and president of Ploughshares, a foundation promoting non-proliferation efforts.

She added: “We’ve also likely increased [Tehran’s] calculus that they will seek nuclear weapons to prevent the very kind of attack we’ve just witnessed.”

According to the IAEA, about 200kg of the HEU, enriched to 60% purity, is being kept down deep shafts under a mountain near the city of Isfahan. On the weekend Le Monde published a satellite photograph from June last year of a large truck at a tunnel entrance at the Isfahan site carrying blue containers, which the Bulletin of the Atomic Scientists assessed most likely contained HEU.

Trump was briefed over the past week on a Pentagon proposal he had requested to secure and extract the HEU stockpile, according to the Washington Post.

The operation would have involved taking control of an area in Iran’s mountainous interior, flying in excavation equipment and building a runway for cargo planes to fly the HEU out of the country, the report said. It would have taken hundreds if not thousands of troops several weeks, exposing them to high risks. Trump’s remarks on Wednesday suggested he had judged the risks to be too high.

The HEU stockpile itself is the consequence of Trump’s decision, in 2018 during his first term, to withdraw from a multilateral nuclear deal agreed three years earlier. That agreement limited the Iranian uranium stockpile to less than 4% enriched. Iran only began making 60% HEU after the agreement fell apart.

“The comment that you can just not worry about the material because you can see it from satellites really fundamentally misunderstands how to manage nuclear risk,” Belcher said. “The issue isn’t just whether we can see the material, it’s whether we can verify, secure and constrain it. And in order to do that, you need diplomacy, inspections and sustained international cooperation.”