China Mobile Ltd, the world's biggest phone company by subscribers, said Monday its deputy chairman is under investigation for unspecified offenses.

Zhang Chunjiang is being investigated "due to suspected serious personal violations," the company said in a statement issued through the Hong Kong stock exchange, where its shares are traded. It gave no details.

Zhang is vice chairman and an executive director of the company. Zhang joined China Mobile in May 2008 after serving as chairman of China Netcom Ltd, a fixed-line carrier that was merged with another phone company in an industry overhaul last year. He also is a former director of the telecoms department of the Chinese ministry that oversees the industry.

China Mobile reported a profit of 83.9 billion yuan ($12.3 billion) for the first nine months of this year but said earnings growth was slowing due to competition. The company says it has 518 million mobile accounts.