Three prominent allies of China's former security chief Zhou Yongkang are under investigation for corruption, state prosecutors said Monday, as the noose appears to tighten around one of the country's most powerful men.

China's top prosecutor's office announced bribery probes into former vice minister of public security Li Dongsheng and Jiang Jiemin, formerly a top regulator of state-owned enterprises, on its website.

It is the latest sign that Zhou, until 2012 a member of the Politburo Standing Committee, China's most powerful body, is being targeted in a corruption investigation.

Also under investigation for taking bribes is Wang Yongchun, formerly a senior manager at China's biggest oil company, China National Petroleum Corporation, the prosecutor's office said in a statement without giving details.

Zhou, who amassed huge power before his retirement from the Politburo Standing Committee in late 2012, has not been seen in public for months.

He had extensive connections in the state-run oil industry and the public security apparatus which he headed until his retirement. Both sectors have been targeted for investigation by the ruling Communist Party.

Several reports have cited sources close to Chinese leaders as saying that Zhou has also been targeted, though there has so far been no public confirmation.

If the investigation into Zhou is confirmed, it would be the first time in decades that such a high-ranking figure has been targeted in a formal inquiry.

President Xi Jinping has vowed to crackdown on endemic graft but critics say he is unlikely to succeed without more fundamental reforms such as greater press freedoms and and independent courts.

The government has also stifled an independent anti-corruption movement, jailing about 10 activists who had been involved in small-scale protests calling for government officials to disclose their financial assets.