President Donald Trump says the United States will not permit Iran to keep its stockpile of highly enriched uranium.

However, Iranian Supreme Leader Mojtaba Khamenei has issued a directive that Iran’s enriched uranium should not be sent abroad, the Reuters news agency reported on Thursday, quoting two unnamed senior Iranian sources.

The future of the estimated 440kg (970lb) of uranium enriched to 60 percent that Iran is believed to be holding remains a chief sticking point in peace negotiations between the US and Iran.

While uranium enriched to 60 percent is still far short of the 90 percent required for weapons-grade material, it is the point at which it becomes much quicker to reach the required 90 percent, nuclear experts said.

But even if Iran were to agree to transferring it, can highly enriched uranium be moved between countries safely?

Here’s what we know:

What have Trump and Khamenei said about the enriched uranium stockpile?

“We will get it. We don’t need it. We don’t want it. We’ll probably destroy it after we get it, but we’re not going to let them have it,” Trump said about Iran’s enriched uranium stockpile when he spoke to reporters at the White House on Thursday.

However, the same day, Reuters reported that Khamenei had issued a directive prohibiting the removal of the uranium.

Reuters additionally reported, quoting unnamed Israeli officials, that Trump had assured Israel that Iran’s enriched uranium stockpile would be sent out of Iran and any peace deal would include a clause on this.

“The supreme leader’s directive and the consensus within the establishment are that the stockpile of enriched uranium should not leave the country,” Reuters reported, quoting one of the two Iranian sources, who spoke on condition of anonymity because of the sensitivity of the matter.

What do we know about Iran’s enriched uranium?

Tehran has said for years that its nuclear programme is for civilian purposes only and it does not intend to build nuclear weapons. It signed a deal with the US and other governments in 2015 to limit its nuclear programme in exchange for sanctions relief, but Trump withdrew from the landmark deal in 2018 and slapped sanctions back on Iran despite international inspectors stating that Iran had stuck to its side of the deal.

As a result of the US withdrawal from that agreement – known as the Joint Comprehensive Plan of Action (JCPOA) – and the bombing of Iran’s Natanz nuclear facility in 2021, which Iran blamed on Israel, it decided to enrich uranium from the 3.67 percent allowed under the 2015 deal for nuclear power development purposes to almost 60 percent.

Iran is now believed to be holding about 440kg (970lb) of uranium enriched to 60 percent. A 90 percent threshold of enriched uranium is needed to produce a nuclear weapon.

In theory, this amount of enriched uranium – should it be enriched to 90 percent – is enough to produce more than 10 nuclear warheads, International Atomic Energy Agency (IAEA) chief Rafael Grossi told Al Jazeera in early March.

Almost all of Iran’s stockpile is thought to be in the form of hexafluoride gas, which can be stored in small canisters, each about the size of a scuba tank. This is spun in centrifuges to increase the proportion of uranium-235, the isotope that can support nuclear fission chain reactions.

Most of Iran’s enriched uranium is believed to be lying underground, beneath the rubble of Iran’s nuclear facilities that were bombed by the US and Israel last year during the 12-day Iran-Israel war. In June, Trump said the US attacks had “obliterated” three Iranian nuclear enrichment facilities at Fordow, Natanz and Isfahan.

But Israel, the US and other Western countries now allege that Iran is seeking or at least preparing the capacity to build nuclear weapons. They argue the 60 percent enrichment level achieved so far is well above what is needed for a civilian nuclear energy programme.

Israeli Prime Minister Benjamin Netanyahu said the current war will not be considered over until Iran’s enriched uranium is removed, Tehran halts support for its proxy armed groups in the region and its ballistic missile capabilities are dismantled.

What could happen to Iran’s enriched uranium?

The US wants this stock to be handed over to it, but Iran was reportedly willing to consider transferring it only to a third party. Now, Khamenei is understood to have issued an order prohibiting its removal at all.

Iranian Foreign Minister Abbas Araghchi told reporters on the sidelines of a meeting of foreign ministers from BRICS nations in New Delhi this month that Iran and the US had reached a “deadlock” on the question of Iran’s “enriched material”.

As a result, he said, the topic is being “postponed” until later stages in the talks. “For the time being, it is not under discussion, it’s not under negotiation, but we will come to that subject in later stages.”

Meanwhile, news reports have suggested that on February 26 during informal negotiations with the US in Geneva two days before the US and Israel launched attacks on Iran, Tehran offered to “downblend” the stockpile from 60 percent enrichment to 3.67 percent in an irreversible process.

“What is likely to happen is that Iran, with monitoring and inspection of the IAEA, will dilute its uranium enriched at 60 percent to enrichment around 3 to 4 percent. This is the most likely scenario,” Muhammad Sahimi, a professor of chemical engineering and materials science at the University of Southern California, said by email to Al Jazeera.

Can enriched uranium be transferred safely?

Uranium hexafluoride gas is extremely dangerous. If released, it can form highly toxic and corrosive fluoride compounds that are deadly when inhaled and can burn the skin.

The IAEA has specific protocols in place to safely transport enriched uranium. According to the nuclear watchdog’s website, enriched uranium hexafluoride can be transported in type 30B containers, which are heavily fortified, standardised steel cylinders. These are specially designed to withstand high pressure and heat.

The IAEA also said these cylinders are deliberately built in a small size to “avoid criticality risks”. In this context, “criticality” means an uncontrolled nuclear chain reaction that releases energy and radiation very quickly.

“If the uranium is to be transferred to another country, it can be done safely,” Sahimi confirmed.

Has enriched uranium been transferred in the past?

The US exported highly enriched uranium to Canada for medical‑isotope production from the mid‑1980s but progressively phased out those shipments as producers converted to low‑enriched uranium. By the mid‑2010s, Washington had authorised what it described as the final exports, and in 2021, the US Department of Energy announced it would no longer supply enriched uranium for medical‑isotope production, saying global markets had successfully shifted to low‑enriched uranium.

After the Cold War, US forces flew about 600kg (1,323lb) of weapons-grade uranium out of Kazakhstan to the US in 1994 in a covert operation called Project Sapphire to remove nuclear material left over from the Soviet Union.

The teams involved in this transport worked 12-hour shifts six days a week for four weeks just to move the material safely from a metallurgical plant to a local airport, according to the Center for Arms Control and Non-Proliferation.