The incident happened in the town of Yaqoobpur in Uttar Pradesh state, the Press Trust of India news agency said, quoting deputy superintendent of police Ashok Kumar Singh.
A loud explosion was heard after the firecrackers caught fire, killing eight on the spot and injuring 12 others, it added.
The crackers were being manufactured illegally ahead of Diwali, the festival of lights when Indians celebrate by letting off fireworks.
In September a major fire at a fireworks factory in southern India killed 38 people.
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