YINCHUAN - A gas poisoning incident at a pharmaceutical plant in northwest China's Ningxia Hui Autonomous Region has left three employees dead and another one severely injured, local authorities said Friday.

The accident took place at about 9 p.m. Thursday at a wastewater disposal facility owned by Ningxia Tairui Pharmaceutical Co. Ltd. and located in the region's Yongning County, said Ding Jianyi, head of the county.

An initial investigation showed that a broken wastewater pipe leaked hydrogen sulfide, poisoning several people, according to the region's Work Safety Administration.

Ten people, including three rescuers, were poisoned by the gas. One worker was severely poisoned and transferred to an intensive care unit, according to the administration.

The plant's operations have been suspended. Tests conducted by local environmental authorities came back normal, according to the administration.