The explosion tore through two factory buildings of Changtu Safe Environment Color and Noise Co Ltd mid-morning on Tuesday in Shuangmiaozi township, Liaoning province, Xinhua news agency reported. 

The blast occurred when workers were producing fireworks for year-end celebrations on Wednesday and for the annual Lunar New Year holiday, only weeks away. 

The two factories covering about 300 square metres were flattened by the explosion. A four-storey office building was badly damaged along with other buildings nearby, the report said. Flames engulfed the site after the blast. 

Provincial-level police officials were investigating the cause
of the disaster, it said. China is the world's biggest producer of firecrackers and fireworks, with people in rural areas often illegally making them in their homes under unsafe conditions. 

Chinese authorities have repeatedly vowed to crack down on illegal and hazardous fireworks factories, but accidents continue to happen.