China's corruption inquiry into Jiang Jiemin, the highest-ranking official to be caught in a crackdown under the new leadership, will focus on a giant state oil firm, state media said Monday.

An earlier Hong Kong report had highlighted an even more senior figure linked to China National Petroleum Corp (CNPC), the country's biggest oil producer.

Jiang worked for decades in China's petroleum industry and rose to become chairman of CNPC.

The China Daily quoted a source close to the firm as saying the investigation into Jiang 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 body, the Politburo Standing Committee -- will also face a corruption inquiry, the Hong Kong-based South China Morning Post newspaper reported last week.

If confirmed, Zhou -- who retired from the standing committee 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 from the southwestern province of Sichuan, which he ran from 1999 to 2002, have come under investigation.

Analysts said several different factors could be involved behind the scenes, including political infighting within the factionalised ruling Communist Party.

Whatever the underlying motive, authorities would be likely to tout an inquiry into Zhou as proving their anti-corruption sincerity, said Jean-Pierre Cabestan, a political scientist at Hong Kong Baptist University.

If the leadership wants to show "everyone can be investigated and prosecuted... I think Zhou Yongkang is a good case", he said.

"Whether they can convince the public is another story, of course, because at that level all the cases are also very political."

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

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

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

Xi Jinping -- who took office as party chief last November and as President in March -- has warned corruption could destroy the party and has threatened to expose high-ranking officials, or "tigers", along with low-level "flies".

A series of officials has 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" -- a phrase which generally refers to corruption.

Bo Xilai, who sat on the 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".