The vice chairman of China's biggest state-owned oil producer, PetroChina Ltd., is under investigation by the ruling Communist Party amid an anti-graft probe of the company.

The party's discipline body says it is investigating Liao Yongyuan for possible "serious violations of the law." It gave no details but a PetroChina statement said Liao was suspected of violating discipline, the party's term for corruption.

PetroChina has been a focus of an anti-corruption campaign led by President Xi Jinping. A number of executives at the company and its parent, China National Petroleum Corp., have been detained.