Fifteen senior officials at ministerial-level and above have been investigated on allegations of corruption in the first half of this year. The officials, 13 at ministerial-level and two at vice national level, were probed for suspected "grave violations of discipline and law," according to the Central Commission for Discipline Inspection of the Communist Party of China, the country's top anti-graft watchdog.

The latest case came on Monday, when Xu Caihou, former vice-chairman of China's Central Military Commission, was expelled from the Party on bribery allegations. He is a former member of the top-ruling Political Bureau of the CPC Central Committee. There are 25 members in the bureau.

Apart from Xu, another vice national level official to come under investigation is Su Rong, 66, former vice-chairman of the Chinese People's Political Consultative Conference National Committee. He was removed from his post at the top advisory body on June 25, 11 days after the CCDI began its probe.

On Friday, Wan Qingliang, Party chief of Guangzhou, the capital of Guangdong province, was announced as being under investigation for corruption. Wan, 50, was deemed a promising official given the fact he was the youngest mayor of Guangzhou city when he got the post at the age of 46.

Ji Wenlin, 48, former vice-governor of Hainan province, is the youngest of the senior officials to come under investigation. He worked as a secretary for a senior official for years before taking the vice-governor post on Jan 31, 2013.

Yang Baohua, 67, former vice- chairman of the political advisory body of Hunan province, is the eldest of the officials under suspicion. He had already been retired from his post for two years when the CCDI announced its investigation of him on May 26.

As of Monday, the CCDI has publicized 314 cases this year in which officials from government bodies, public institutes and State-owned enterprises have been investigated on allegations of corruption.

Those cases involved at least 40 senior managers of SOEs, mostly from monopoly industries such as energy, telecommunications and electricity. Five officials from different levels of the State-owned Assets Supervision and Administration Commission, which is in charge of SOE regulations, were also investigated.

Twenty officials from universities and colleges have been probed, mostly on bribery allegations.

The anti-graft authorities have also enhanced supervision of disciplinary officials, and six officials from anti-graft agencies have been investigated. Wei Jian, director of the No 4 discipline inspection office of the CCDI, who was investigated on May 9, was the highest ranking among those officials.

At least five officials from the press have been also been investigated in the past six months. Among them, the investigation of Guo Zhenxi, director general of China Central Television's finance and economics channel, was of great concern to the public due to the powerful influence of the CCTV.

Zhu Shuzhen, a professor of clean-governance research with Renmin University of China, said more senior officials are likely to be investigated as the top anti-graft watchdog has collected a large number of whistle-blowing tips through dispatching inspectors to local governments.

"Fighting corrupt senior officials is key to anti-graft work since they have overwhelmingly large power and their corrupt activities always result in huge losses of State funds," she said.

In the first half of last year, the CCDI investigated five ministerial-level officials. Anti-graft efforts were strengthened in November 2013 after the Third Plenary Session of the CPC Central Committee, when a reform package was agreed upon by top officials.