China's top anti-graft body said on Thursday is was investigating a former executive at an oil firm owned by state-run China National Petroleum Corp , as the government presses ahead with an anti-corruption campaign targeting the energy sector.

Sun Weidong, the former vice-manager of the Yumen Oilfield Company in western Gansu province, is being investigated on suspicion of seriously violating discipline, the Central Commission for Discipline Inspection said in a one-line statement, using the Communist Party term for corruption.

A CNPC spokesman told Reuters by telephone that he was not aware of the report.

Sun is the second executive at the Yumen Oilfield Company under investigation. In March, the official Xinhua news agency's website reported the oilfield's deputy general manager and chief accountant, Yang Guoling, was being probed for "violating party discipline".

CNPC, the parent of China's biggest oil and gas producer PetroChina , is at the centre of one of the biggest corruption investigations into the Chinese state sector in years.

Five former executives from CNPC and PetroChina are being investigated on graft charges, including Jiang Jiemin, chairman of both entities, who was a vocal proponent of expansion of what he called the national and social responsibilities of state-owned enterprises.

Dozens of government auditors launched a probe into China's State Grid Corp last month, though the utility has said the audits are routine.

President Xi Jinping has vowed to tackle pervasive graft, saying he will go after high-flying "tigers" as well as lowly "flies".