China's former security chief under investigation: Xinhua

China's former security chief Zhou Yongkang is under investigation, the official Xinhua news agency said Tuesday, in a move targeting one of the ruling Communist Party's most powerful men, AFP reports.

Zhou, who retired from China's all-powerful Politburo Standing Committee (PSC) in 2012, is being probed for "serious disciplinary violation", Xinhua said.

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 ruling party's Central Commission for Discipline Inspection (CCDI), its 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 added.

The move is likely to send shockwaves through China's political establishment, as PSC members have long been regarded as untouchable even after retirement.

For months, allies of Zhou in his powerbases 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.