CHONGQING - Four railway construction workers suffocated after inhaling toxic fumes in southwest China's Chongqing municipality, local authorities said Thursday.

The accident occurred at around 7 am Wednesday when the victims were working underground to build a section of a high-speed railway linking Chongqing and Chengdu, capital city of Sichuan province, said a spokesman with Chongqing's work safety bureau.

The four men passed out suddenly during their work and later died in hospital, said the spokesman.

The rail project was undertaken by a subsidiary of China Communications Construction Company, a leading state-owned corporation.

The cause of the accident is under investigation.