Four workers were killed by an unknown gas when they were repairing heating pipelines in north China's Inner Mongolia autonomous region, the local work safety watchdog said Wednesday.
The accident happened around 11 a.m. when two workers cut off a pipe in a well and were poisoned by the gas which leaked out of the pipe at Ulan Qab city's Jining New District, Xinhua reported.
Another two workers who entered the well to rescue them were also poisoned and died later despite medical efforts, said a statement from the watchdog.
The cause of the accident is being probed.