China's former security chief is under investigation, the official Xinhua news agency has announced, in a move targeting one of the ruling Communist Party's most powerful men.

Zhou Yongkang, who retired from China's all-powerful Politburo standing committee in 2012, is being investigated for "serious disciplinary violation", Xinhua said on Wednesday.

The term is usually used to refer to corruption.

With the official announcement of the long-rumoured probe, Zhou becomes the most senior member of the Communist Party to be investigated since the infamous Gang of Four - a faction that included the widow of founding leader Mao Zedong - in 1980.

Zhou was once an ally of fallen political star Bo Xilai.

The investigation will be conducted by the party's internal watchdog, Xinhua said.

The decision was made in accordance with the ruling party's constitution, and the "discipline inspection authority's case investigation regulation", it said.

Standing committee members have long been regarded as untouchable, even after retirement.

For months, allies of Zhou in his power bases in the southwestern province of Sichuan and China's state-owned oil giant CNPC have been targeted one by one by the ruling party's internal watchdog.

Source: AFP