Dozens of senior executives in State-owned enterprises have been investigated on corruption charges in the first round of an inspection this year by the country's top anti-graft authority.

The Central Commission for Disciplinary Inspection held 19 top managers from SOEs during its first disciplinary tour from late February to the end of April.

Thirteen teams were sent to the giant corporations, with each team reviewing two companies, including China National Petroleum Corp, China National Offshore Oil Corp, China Huaneng Group, State Grid Corp of China and China Mobile Communications Corp.

The number of enterprises under scrutiny was almost double that of previous inspections. Last year, such inspections investigated more than 70 executives.

Companies in the oil and refinery sector are among those hardest-hit since the anti-corruption inspections intensified in SOEs with five top managers held from the three largest oil enterprises in China.

Liao Yongyuan, vice chairman of PetroChina Ltd and general manager of its parent company, CNPC, was placed under investigation on March 16 on suspicion of committing "serious violations of the law".

In addition to Liao, a 30-year-veteran and the key figure in oil exploration in Tarim Oilfield, An Wenhua, deputy general manager of PetroChina Tarim Oilfield Company, and Jia Dong, the oil field's chief accountant, also were placed under investigation in March.

Prior to the inspection, Wang Lixin, head of the conglomerate's disciplinary department, was investigated on suspicion of corruption in December.

About 50 executives have been held for investigation since CNPC was probed by the anti-graft authority in the middle of 2012.

Jiang Jiemin, a former regulator of State-owned assets who was investigated in September 2013, was head of the corporation.

Zhou Yongkang, China's former security chief, his predecessor at the corporation, also was placed under investigation.

Of the 19 top managers held in the latest round, seven were executives in SOEs in the electric power sector and four in the telecommunication sector.

The SOEs saw their profits rise by 3.4 percent last year to about 2.48 trillion yuan ($40 billion), but their debts increased by 12.2 percent to 6.66 trillion yuan, according to the Ministry of Finance.

A far-reaching plan to merge SOEs made news this week.

The number of major SOEs will reportedly be reduced from 112 to around 40. However, no timeline or details about specific mergers were released.