China has placed a former close aide to the country's ex-chief of internal security Zhou Yongkang under investigation, authorities said, heightening speculation that the net is tightening around his former patron.

Ji Wenlin, a vice governor of the southern island of Hainan, is being investigated for "suspected severe violations of disciplines and laws", the Central Commission for Discipline Inspection (CCDI) -- the party's internal graft watchdog -- said in a one-sentence announcement on its website Tuesday.

The phrase is usually an indication of alleged corruption.

Ji, 47, was a secretary and close aide to Zhou when he was land resources minister and then party chief of Sichuan province in the southwest in 1998-2002, according to multiple Chinese media reports.

It is the latest sign of moves against Zhou, who amassed huge power during his time as China's security chief but retired from his position as a member of the Communist Party's all-powerful Politburo Standing Committee (PSC) in late 2012.

So far 21 officials at vice-ministerial level or above have fallen since the once-in-a-decade power transition that anointed Xi Jinping as the party's general secretary.

Among the 21, at least six are believed to have been proteges of Zhou.

The New York Times in December cited "sources with elite political ties" as saying that Xi had given the go-ahead for a corruption investigation into Zhou himself.

It would be the first time in decades that such a high-ranking figure has been targeted in a formal inquiry, and would send shockwaves through China's elite. PSC members have generally been regarded as untouchable even after retirement.

But the Beijing News even mentioned Zhou by name in its report about Ji's fall on Wednesday.

Other media outlets said that Ji was the secretary of "a former Politburo Standing Committee member" and that he "crossed paths" with several ousted officials in Sichuan.

Xi, who became head of state in March last year, has warned that corruption could destroy the party and vowed to crack down on high-ranking officials, described as "tigers", along with low-level "flies".

Separately, the CCDI announced Wednesday that Zhu Zuoli, a vice chairman of the northern province of Shaanxi's Political Consultative Conference -- a debating chamber that is part of the Communist Party-controlled governmental structure -- was under investigation for severe violations of discipline.

Recent high-profile cases include the sacking of Jiang Jiemin, head of China's state-owned assets watchdog, and Li Dongsheng, formerly a vice minister of public security, both of them allies of Zhou.

But critics say no systemic reforms have been introduced to increase transparency to help fight endemic graft.