The inquiry into the highest-ranking Chinese official to be investigated for graft in a current crackdown will focus on an oil firm he headed linked to other senior figures, state-run media said Monday.

Jiang Jiemin worked for decades in China's petroleum industry, rising to become chairman of state-owned China National Petroleum Corp, the country's biggest oil producer.

The China Daily quoted a source close to the firm as saying the investigation was "related to alleged graft when he was head of the company," possibly tied to oilfield contracts and "ill-gotten payouts".

Another former top CNPC official, Zhou Yongkang, who went on to become China's security tsar and a member of its highest-ruling body, the Politburo Standing Committee (PSC), is also to face a corruption inquiry, the Hong Kong-based South China Morning Post newspaper reported last week citing unspecified sources.

If confirmed, Zhou -- who retired from the PSC in a leadership transition last year -- would be the most senior official investigated in China for decades.

Rumours about the possibility have swirled for months as senior figures from CNPC, which Zhou headed from 1996 to 1998, and the southwest province of Sichuan, which he ran from 1999 to 2002, have come under investigation.

Four senior CNPC executives are already being investigated, officials said last week.

Jiang is the first member of the ruling Communist Party's current 205-person Central Committee to face investigation, the China Daily said.

In his most recent post Jiang headed the State-owned Assets Supervision and Administration Commission, which oversees China's many powerful state-owned enterprises.

President Xi Jinping has warned corruption could destroy the party and threatened to expose high-ranking officials, or "tigers", along with low-level "flies".

A raft of officials have since come under investigation for alleged corruption, including Liu Tienan, former deputy director of the influential National Development and Reform Commission.

Li Chuncheng, who became mayor of Sichuan's capital Chengdu during Zhou's time in the province, lost his post in December for "serious violations of discipline", which generally refers to corruption.

Bo Xilai, who sat on the Communist Party's 25-member Politburo and whose spectacular trial for bribery and other charges ended last week, is also reported to be an ally of Zhou.

There are deep factional divisions within the party but the state-run Global Times trumpeted the Jiang inquiry in a commentary, saying "it reinforces public confidence in the country's system for netting criminals".