LANZHOU - Carbon monoxide has been blamed for four deaths of workers in a cement plant in northwest China's Gansu Province on Wednesday, local officials said.

Seven workers were poisoned by carbon monoxide at 6:40 p.m. when they were adding steel balls into a raw mill at the cement plant in Pingliang City, according to officials from the city government and the provincial work safety bureau.

The poisoned workers were rushed to hospital, but four people died despite medical efforts. The other three have been out of danger after receiving medical treatment.

Preliminary investigation shows too much soot in the raw mill may be the cause of the accident. Further investigation is still underway.