Sinopec, China's biggest oil refiner, said Friday it is cooperating with a Hong Kong graft investigation after reports accused the company of bribery relating to its petrol stations in the city.

Executives at the state-owned company are suspected of taking bribes from prospective contractors who were bidding to run petrol stations in the Kowloon area of Hong Kong, the Chinese-language Apple Daily newspaper reported, citing an anonymous source.

The Apple Daily said two senior executives from the company's Hong Kong unit were questioned by the city's Independent Commission Against Corruption, without elaborating on how many stations were included in the inquiry or the amount of money involved.

"The company is cooperating fully with the relevant authority," Sinopec said in a statement sent to AFP on Friday.

"The enquiries relate to the individual conduct of the individuals concerned," Sinopec said.

"It has always been the company's policy to run its business in line with Hong Kong's core values and it will not tolerate any illegal and corrupt conduct," it said.

The company, which owns 42 filling stations in the city, said its operations were not affected.

The oil giant said in January that it would pay compensation over a November pipeline explosion at its facility in the city of Qingdao that killed dozens of people and caused losses of more than $100 million.

Its former head, Chen Tonghai, was handed a suspended death sentence in 2009 after being found guilty of corrupt practices over many years.