At least 10 people were killed and seven others injured following an explosion at a fireworks plant in China's Henan province on Thursday, a media report said.The injured, of whom five sustained critical injuries, were being treated in various hospitals in Tongxu county of Kaifeng city, Xinhua news agency reported.The blast occurred at 10.48 a.m. (local time) , the local work safety authority said.An earlier report had stated that the blast took place at an illegal fireworks plant. Initial reports also placed the toll at five.The cause of the blast was being investigated.