The cause of the blast in Muzaffarpur was being investigated, said police officer Ram Naresh Paswan. He said the death toll was likely to rise as four of the injured were critical.
Panic gripped the area as the powerful explosion shook and damaged homes and factories in the Bela industrial area, media reports said.
Poor safety standards are a frequent cause of fires and explosions in India.
Last year, a major fire and a powerful blast rocked a warehouse storing chemicals, killing 12 people near Ahmedabad in western Gujarat state.