A huge explosion at a military base in south-western Cambodia that killed 20 soldiers and injured many others was an accident caused by a “technical issue” stemming from old ammunition that was being moved, the defence ministry said.

The army said Saturday’s blast was believed to have been an accident caused by mishandling of degraded ammunition by troops.

The defence ministry’s statement followed an allegation levelled by an opposition politician-in-exile suggesting that the explosion had been an attack.

The day after the explosion, exiled opposition leader Sam Rainsy claimed in a post on his Facebook page that an armed group of anti-government dissidents had caused the blast, but cited no evidence to support his claim.

The defence ministry said that while there had been recent social media posts by “a group of extremists”, the ministry wanted to inform media that the investigation had concluded the blast had been an accident.

It warned that anyone publishing untrue information about the blast could face prosecution.

Army spokesperson Maj Gen Mao Phalla said soldiers were transferring ammunition into a storage facility when the blast occurred, killing 20 instantly.

He said another 11 people, including soldiers and nearby villagers, were slightly injured from the damaged building’s debris, not shrapnel.

The blast in Kampong Speu province also destroyed military vehicles and four buildings at the base, and damaged homes in a village.