HANOI (Reuters) - Prosecutors in Vietnam on Thursday said they were seeking the death sentence in an embezzlement case against a former chairman of state energy firm PetroVietnam, as the communist country steps up one of its biggest corruption crackdowns. Some high-ranking political officials have been punished as investigations widen into PetroVietnam and the banking sector, with dozens of banking and energy officials facing trial on charges such as embezzlement, mismanagement and abuse of power. In a statement, the Supreme People s Procuracy of Vietnam said it had sought a death sentence for the former chairman, Nguyen Xuan Son, on charges that include wrongdoing with serious economic consequences and abuse of power to usurp assets. It urged  an overall penalty of death , listing punishments such as a jail term of 16 to 18 years for flouting state rules on economic management and life imprisonment for abuse of power, before seeking the  death sentence for embezzlement . In 2009, PetroVietnam acquired an 800-billion-dong ($35-million) stake in Ocean Group s banking unit, Ocean Bank, which had to be completely written off in 2015, when the central bank took it over at no cost.  Son could not be reached for comment as he is on trial, and Reuters could not immediately reach his lawyer. Prosecutors also sought life imprisonment for Ocean Group s founder, tycoon Ha Van Tham on charges ranging from embezzlement to abuse of power, the statement said, adding that dozens of other Ocean Bank staff could also face years in jail. Ocean Group, which has interests in real estate, finance, hotels and infrastructure, said it had no comment on the sentence sought for Tham, who cannot be reached as he is still on trial. Police opened three new cases against state firm units, among them Russian joint venture Vietsovpetro, Vietnam s sole refinery operator Binh Son Refining and Petrochemical, and PetroVietnam Exploration Production Corp. All three cases focus on alleged abuse of power to usurp assets and are linked to violations at Ocean Bank, Vietnam s police said in a statement on their website on Thursday. Police added another accusation of abuse of power against PetroVietnam s vice general director Ninh Van Quynh, they added, following his arrest and prosecution this month for alleged wrongdoing. Quynh could not be contacted for comment on the fresh charge as he is on trial. Last week a former central bank governor was prosecuted for alleged lack of responsibility. Dinh La Thang, a former PetroVietnam chairman, was removed from the powerful politburo and last month, a vice trade minister was sacked. 