BEIJING (AFP) - China is investigating a top executive at its largest oil refiner, the anti-corruption watchdog said Monday, as a prolonged graft crackdown targets more state-owned companies.

Wang Tianpu, general manager of Sinopec, is being probed for "serious law and discipline violations", a common euphemism for corruption, the Communist Party's Central Commission for Discipline Inspection (CCDI) said in a one-line statement on its website.

Wang is also a member of the company's board.

The investigation comes ahead of the trial of former security chief Zhou Yongkang and his allies in China's oil sector, although it is unclear if the two are related.

President Xi Jinping launched a much-publicised drive against corruption after he came to power two years ago, vowing to target both high-level "tigers" and low-ranking "flies".

The CCDI said early this year that it plans to investigate all major state-owned enterprises.

Analysts say China has failed to implement institutional safeguards against corruption, such as an independent judiciary and free media, leaving anti-graft campaigns subject to political influence.