Some 2.5 tonnes of natural uranium stored in a site in war-torn Libya has gone missing, the United Nations nuclear watchdog says, raising safety and proliferation concerns.

Natural uranium can't immediately be used for energy production or bomb fuel, as the enrichment process typically requires the metal to be converted into a gas, then later spun in centrifuges to reach the enrichment levels needed.

However, each tonne of natural uranium — if obtained by a group with the technological means and resources — can be refined to 5.6 kilograms of weapons-grade material over time, experts say.

That makes finding the missing metal important for nonproliferation experts.

In a statement, the Vienna-based International Atomic Energy Agency (IAEA) said its director-general, Rafael Mariano Grossi, informed member states on Wednesday about the missing uranium.

The IAEA's statement, however, did not contain many details.

On Tuesday, "agency safeguards inspectors found that 10 drums containing approximately 2.5 tons of natural uranium in the form of uranium ore concentrate were not present as previously declared at a location in the state of Libya," the statement said.

Reuters first reported on the IAEA warning about the missing Libyan uranium, saying the IAEA told members that reaching the site, which is not under government control, required "complex logistics".

The IAEA declined to offer more details on the missing uranium. However, its acknowledgement the uranium went missing at a "previously declared site" narrows the possibilities.

Uranium left over from Gaddafi regime weapons program

One such declared site is Sabha, some 660 kilometres south-east of Libya's capital, Tripoli, in the country's lawless southern reaches of the Sahara Desert.

There, Libya, under dictator Moamar Gaddafi, stored thousands of barrels of so-called yellowcake uranium for a once-planned uranium conversion facility that was never built, as part of his decades-long secret weapons program.

Estimates put the Libyan stockpile at some 1,000 metric tonnes of yellowcake uranium under the late Mr Gaddafi, who declared his nascent nuclear weapons program to the world in 2003 after the US-led invasion of Iraq.

While inspectors removed the last of the enriched uranium from Libya in 2009, the yellowcake remained behind, with the UN in 2013 estimating some 6,400 barrels of it were stored at Sabha.

American officials had worried Iran could try to purchase the uranium from Libya, something Mr Gaddafi's top civilian nuclear official tried to reassure the US about, according to a 2009 diplomatic cable published by WikiLeaks.

"Stressing that Libya viewed the question as primarily a commercial one, [the official] noted that prices for uranium yellowcake on the world market had been increasing, and that Libya wanted to maximise its profit by properly timing the sale of its stockpile," then ambassador Gene A Cretz wrote.

But the 2011 Arab Spring saw rebels topple Mr Gaddafi and ultimately kill him. Libya has had little peace since then.

Since 2014, political control has been split between rival eastern and western factions, with the last major bout of conflict ending in 2020.

Sabha grew increasingly lawless, with African migrants crossing Libya saying some had been sold as slaves in the city, the UN reported.

In recent years, Sabha has largely been under the control of the self-styled Libyan National Army, headed by Khalifa Hifter.

The general, who is widely believed to have worked with the CIA during his time in exile during Mr Gaddafi's era, has been battling for control of Libya against a Tripoli-based government.

A spokesman for Mr Hifter declined to answer questions from the Associated Press. Chadian rebel forces have also had a presence in the southern city over recent years.

Libya's interim government, put in place in early 2021 through a UN-backed peace plan, was only supposed to last until an election scheduled for December of that year. However, that election has still not been held.

AP/Reuters