A former high-ranking Chinese military leader is under investigation for allegedly taking bribes, a Hong Kong-based watchdog said on Tuesday, coming on the back of the nation's much-publicised anti-corruption drive.

Guo Boxiong, 72, a former vice chairman of China's Central Military Commission, is suspected of "helping others gain promotions by receiving bribes", the Information Center for Human Rights and Democracy said in a report.

The report said the military ran on a "dual leadership" system where a person's promotion must be approved by the vice chairmen of the commission.

"The person seeking to issue bribes must give presents to both people," the report said, without citing sources.

Guo's colleague Xu Caihou, also a former vice chairman of the commission, was stripped of his party membership over graft in June when his case was handed over to prosecutors.

Xu, 71, was the first former member of the Communist Party's elite 25-strong Politburo to fall in President Xi Jinping's crackdown on graft.

The People's Liberation Army's influence in domestic affairs has waned since the days of Communist China's founding father Mao Zedong, but it remains a political force to be reckoned with and has at the same time built up a vast network of business interests.

The dramatic expulsion of Xu is an assertion of political control over the powerful and wealthy military by Xi, who intended to send the PLA a clear message, analysts have said.

Xi took office as president in 2012 warning that graft could destroy the party and vowing to root out corrupt officials from high-ranking "tigers" to low-level "flies".