Amid rising tension with the U.S., media reports indicate that the Houthis have acquired U.S. weapons, and that half a million weapons obtained by the Taliban in Afghanistan have been lost, sold or smuggled to militant groups.
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.
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 Houthis, were accessing Taliban-captured weapons or buying them on the black market.
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.
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 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 "unaccounted" for.