Police initially said the blast, in the town of Kazanluk, 230 km (144 miles) east of the capital Sofia, triggered a fire that was quickly put out. But Nikolov said there was no fire and no danger of any further explosions.
It was too early to say what the reason was for the explosion, he said.
There have been several other accidents in the arms industry in the Black Sea state. In 2014, a series of powerful blasts at a Bulgarian explosives plant killed 15 employees in the village of Gorni Lom, some 145 km (90 miles) northwest of Sofia.
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