Three executives of the listed arm of China's biggest oil producer are being investigated for "violations of discipline", the government said Tuesday, a phrase which typically refers to corruption.

PetroChina vice presidents Li Hualin and Ran Xinquan as well as chief geologist Wang Daofu were under investigation by authorities, said the company, which is listed in both Hong Kong and Shanghai.

The three had resigned from their positions, the firm said, adding its business operations were unaffected. It gave no reason for the investigation.

PetroChina has a net asset value of more than one trillion yuan ($160 billion), according to the Hong Kong Stock Exchange, where trading in the company's shares was suspended Tuesday, as it was in Shanghai.

China's State-owned Assets Supervision and Administration Commission, which oversees state firms, confirmed the investigation in a separate statement, saying it was into "serious violations of discipline".

Li was also a deputy general manager of PetroChina's parent, China National Petroleum Corp (CNPC).

State media announced a day ago that Wang Yongchun, one of CNPC's vice presidents, was also under investigation.

The announcements came after the trial of disgraced politician Bo Xilai for bribery, embezzlement and abuse of power ended Monday after five days of hearings.

China's president Xi Jinping has vowed to crack down on corruption at all levels of the government, calling graft a threat to the future of the ruling Communist party.

PetroChina said its shares would resume trading on Wednesday.