The explosion was in the village of Bajgah, near the town of Khenjan, some 120km (80 miles) north of Kabul.
An interior ministry spokesman said the whole area around the munitions dump had been destroyed.
He said the dump belonged to a former militia commander who said he had disarmed under a government scheme.
The spokesman, Lutfallah Mashal, said most of the injured had been evacuated but that casualties were expected to rise.
