Half a million weapons obtained by the Taliban in Afghanistan have been lost, sold or smuggled to militant groups, sources have told the BBC – with the UN believing that some have fallen into the hands of al-Qaeda affiliates.

The Taliban took control of around one million weapons and pieces of military equipment – which had mostly been funded by the US – when it regained control of Afghanistan in 2021, according to a former Afghan official who spoke to the BBC anonymously.

As the Taliban advanced through Afghanistan in 2021, many Afghan soldiers surrendered or fled, abandoning their weapons and vehicles. Some equipment was simply left behind by US forces.

The cache included American-made firearms, such as M4 and M16 rifles, as well as other older weapons in Afghan possession that had been left behind from decades of conflict.

Sources have told the BBC that, at the closed-door UN Security Council's Sanctions Committee in Doha late last year, the Taliban admitted that at least half of this equipment is now "unaccounted" for.

A person from the committee said they had verified with other sources that the whereabouts of half a million items was unknown.

In a report in February, the UN stated that al-Qaeda affiliates, including Tehreek-e-Taliban Pakistan, the Islamic Movement of Uzbekistan, the East Turkestan Islamic Movement, and Yemen's Ansarullah movement, were accessing Taliban-captured weapons or buying them on the black market.

The BBC put this to Hamdullah Fitrat, deputy spokesperson for the Taliban government, who told the BBC it took the protection and storage of weapons very seriously.

"All light and heavy weapons are securely stored. We strongly reject claims of smuggling or loss," he said.

A 2023 UN report said the Taliban allowed local commanders to retain 20% of seized US weapons, and that the black market was thriving as a result. These commanders are affiliated to the Taliban but often have a degree of autonomy in their regions.

The UN noted that the "gifting of weapons is widely practiced between local commanders and fighters to consolidate power. The black market remains a rich source of weaponry for the Taliban".

A former journalist in the city of Kandahar told the BBC that an open arms market existed there for a year after the Taliban takeover, but has since gone underground via the messaging service WhatsApp. On it, wealthy individuals and local commanders trade new and used US weapons and equipment – mostly the weapons left by US-backed forces.

The number of weapons recorded by the US body tasked with overseeing Afghan reconstruction projects, known as Sigar, is lower than those cited by our sources, but in a 2022 report it acknowledged it was unable to get accurate information.

The reason given for this was that equipment has been funded and supplied by various US departments and organisations over the years.

Sigar added that there had been "shortfalls and issues with DoD's [Department of Defense] processes for tracking equipment in Afghanistan" for more than a decade.

It also criticised the State Department, adding: "State provided us limited, inaccurate, and untimely information about the equipment and funds it left behind." The department denied this was the case.

This is very much a political issue, and US President Donald Trump has repeatedly said that he will reclaim weapons from Afghanistan. He said that $85bn (£66bn) of advanced weaponry was left there.

"Afghanistan is one of the biggest sellers of military equipment in the world, you know why? They're selling the equipment that we left," Trump said during his first cabinet meeting of the new administration.

"I want to look into this. If we need to pay them, that's fine, but we want our military equipment back."

The president's figure has been disputed, as money spent in Afghanistan also funded training and salaries. Also, Afghanistan did not feature in the Stockholm International Peace Research Institute's top 25 largest exporters of major arms last year.

In response to Trump's comments, Zabihullah Mujahid, the Taliban's chief spokesperson, told Afghan state TV: "We seized these weapons from the previous administration and will use them to defend the country and counter any threats."

The Taliban regularly parades US weapons, including at Bagram Airfield, which served as the main US-Nato base, and frame them as symbols of victory and legitimacy.

After withdrawing in 2021, the Pentagon claimed US equipment left in Afghanistan was disabled, but the Taliban have since built a capable military using US weapons and gained superiority over rival groups, such as the National Resistance Front and Islamic State Khorasan Province – the regional affiliate of the Islamic State group.

A source from the former Afghan government told the BBC that "hundreds" of unused Humvees, mine-resistant ambush protected vehicles (MRAPs), and Black Hawk helicopters remain in Kandahar warehouses.

The Taliban has showcased some of this captured equipment in propaganda videos, but their ability to operate and maintain advanced machinery, such as Black Hawk helicopters, is limited due to a lack of trained personnel and technical expertise. Much of this sophisticated equipment remains non-operational.

However, the Taliban have been able to utilise more straightforward equipment, like Humvees and small arms, in their operations.

While Donald Trump appears determined to reclaim US weapons from Afghanistan, the former head of Sigar, John Sopko, says such an attempt would be pointless.

At a recent event hosted by the Afghan Institute for Strategic Studies, he said that "the cost would exceed its actual value".

Whether Trump will take any action remains to be seen, but, in the meantime, concerns about the spread of weapons in the region and access by militant groups remain unresolved.