The Chinese government has questioned two more executives from China National Petroleum Corp as part of a wider graft investigation into the state energy giant, two people with knowledge of the matter said.

CNPC and its listed unit PetroChina are at the center of one of the biggest corruption investigations into the Chinese state sector in years.

Wen Qingshan, CNPC's chief accountant, and Wang Lihua, head of PetroChina's oil trading vehicle Chinaoil, were "taken away" by authorities last week, the insiders said. Wang is also CNPC's deputy chief economist.

It was not immediately clear whether the officials were the target of the investigation or assisting in the broader probe.

Two officials at Chinaoil — Zheng Jun, head of crude oil trading, and a company lawyer surnamed Chen — said the report on Wang was "totally incorrect."

"Such a thing that you reported has never happened to President Wang," Zheng. "She is still handling daily management work."

Zheng declined to put Reuters in touch with Wang. Other attempts to reach her were unsuccessful, as were efforts to contact Wen.

One of the two sources said: "Several officials were taken away by the authorities. One is quite senior. He is CNPC's chief accountant Wen Qingshan. The other officials included ... Wang Lihua."

PetroChina spokesman Mao Zefeng declined to comment, although PetroChina told the Hong Kong Stock Exchange that Wen had resigned for personal reasons. In a separate statement, Kunlun Energy, PetroChina's natural gas distribution arm, said Wen had resigned for personal reasons.

Wen was appointed chairman in August, replacing Li Hualin, who the company had said was being investigated.

Zhang Bowen, an executive director, would act as Kunlun Energy chairman, the statement said, adding that the company had applied to the Hong Kong exchange for its shares to resume trading today after they were suspended yesterday.

The Chinese government stunned the energy industry earlier this year with announcements that five former top executives at PetroChina and CNPC were being investigated. They included Jiang Jiemin, former chairman of both.