The blast was caused when a soldier dropped a mortar bomb while loading a truck with weapons surrendered by local fighters, officials told Reuters on Monday.
The incident took place late Sunday night at the village of Aqcha, Mohammad Qoul Zahri, deputy police chief of Jozjan, said.
The soldiers involved were loyal to Abdul Rashid Dostum, a warlord and a defence adviser to Afghani President Hamid Karzai.
"It was a huge explosion that destroyed five of the trucks," Zahri said.
Local television reported that a disarmament ceremony, held after weapons were handed in by paramilitaries across the region, had been attended by British, US and Turkish diplomats. They left before the explosion.
Zahri said the blast was an accident.
“There was no terrorist involvement," Zahri told Reuters. "This was an unfortunate incident. Today General Dostum visited the site and the investigation showed it was an accident.”
The ceremony was held as part of an initiative to disarm more than 100,000 factional fighters across the war-ravaged country.