BEIJING (Reuters) – Three workers were killed and six others injured during a suspected gas leak at a PetroChina-operated refinery in Dalian on Saturday evening, state media said on Monday.

The men working for Henan Yanling Jingshun Petrochemical Machinery Equipment Co Ltd were carrying out maintenance at the West Pacific Petrochemical Corp (WEPEC) plant, in northeast Liaoning province.

They were thought to have been poisoned by hydrogen sulphide, a highly toxic gas found in natural gas and crude petroleum, the China News report said.

A PetroChina spokesperson could not immediately comment on the accident. A WEPEC official said production at the 200,000-barrels per day plant had not been affected by the accident.

The Dalian government is investigating the cause of the incident, the China News report said.