YINCHUAN - A gas poisoning has left three workers dead and another one severely hurt in a pharmaceutical plant in Northwest China's Ningxia Hui autonomous region, local authorities said Friday.

The accident took place at about 9 pm Thursday in a waste water disposal workshop in the Ningxia Tairui Pharmaceutical Co Ltd, located in the Yongning county, said Ding Jianyi, head of the county.

Altogether 11 workers had been exposed to the toxic gas and rushed to hospital.

Local authorities are investigating the cause of the accident, Ding said.