It was not immediately clear what triggered the explosion which caused the collapse of a building at the factory in the town of Makolno, the TVN24 commercial news channel reported quoting fire fighters at the site.
Rescue crews used sniffer dogs to locate other possible victims in the rubble, dangerously laced with gun powder.
A huge white plume of smoke could be seen over a dozen kilometres (miles) away from the site of the blast, which also shattered windows in nearby houses.