Three top executives at Chinese state-owned oil and gas firm PetroChina are under investigation for alleged corrupt practices, as China's Communist government steps up efforts to root out corruption by officials.

The State-Owned Assets Supervision and Administration Commission said it was investigating three senior executives at PetroChina over "serious breaches of discipline".

PetroChina, the largest oil and natural-gas producer in China, released a statement to announce the resignations.

Li Hualin, company vice-president, and secretary to the board, Ran Xinquan, executive director and vice president of the company, and Wang Daofu, chief geologist of the company, are the three departing executives currently under investigation by the authorities.

The firm said the officials resigned with immediate effect "due to personal reasons".

Following the statement, PetroChina shares fell as much as 4.7% during early trading in Hong Kong. In addition, the company's natural gas distribution arm, Kunlun Energy, plunged 12%.

The investigation comes as the company pursues two acquisitions abroad in order to address the shrinking energy resources in the home country. The company is due to buy a stake worth billions of dollars in one of Iraq's largest oil fields from US oil giants Exxon Mobil. It is also looking to acquire a stake in an oil field in Kazakhstan held by the Kazakh government.

Rising Anti-Graft Measures

On Monday, China's Ministry of Supervision said that Wang Yongchun, a top official at PetroChina's parent company, China National Petroleum Corporation, was under investigation. The ministry did not provide further details.

The anti-graft measures comes as part of the ruling Communist Party's high-profile anti-corruption campaign to regain its credibility. President Xi Jinping said that widespread corruption in the country was threatening the survival of the Party.

Liu Tienan, the former deputy head of the National Development and Reform Commission, was removed from his office earlier in August after he was found to have received huge amounts in bribes.

Furthermore, Politburo member Bo Xilai was ousted over corruption allegations, while the state-owned China Mobile Communications removed the head of its Guangdong unit who is being probed by authorities.

In July, former Railways Minister Liu Zhijun was given a suspended death sentence for corruption and abuse of power.