BEIJING (Reuters) - China will prosecute former senior officials of energy giant Sinopec Group and China Southern Airlines as well as a one-time top security official in Tibet for suspected graft, the prosecutor's office said on Monday.

President Xi Jinping launched a sweeping crackdown on deep-rooted corruption after assuming power almost four years ago, ensnaring top politicians, leaders of state enterprises and senior bankers.

In a short statement on its website, the state prosecutor said it had begun proceedings against former Sinopec Group president Wang Tianpu, former China Southern Airlines general manager Si Xianmin, and Le Dake, once a senior security official in restive Tibet.

It was not possible to reach any of the three for comment and unclear who their lawyers are.

A Sinopec spokesman said the firm supported the case against Wang, as corruption was tolerated by neither the ruling Communist Party nor the state, and contradicted the company's core values.

China Southern officials could not immediately be reached for comment.

The prosecutor said all three are suspected of taking bribes and abusing their positions for personal advantage.

It added that the prosecutor had protected the rights of the accused, in line with the law, and heard the opinions of their defense teams. The statement gave no details.

The announcement is the next step in the legal process before the three face trial, which could still be several months away and is likely to happen behind closed doors.

The three have already been investigated by the party's internal anti-corruption watchdog and will almost certainly be found guilty, as Chinese courts are controlled by the party and do not challenge its accusations.

(Reporting by Beijing Monitoring Desk and Chen Aizhu; Writing by Ben Blanchard; Editing by Kenneth Maxwell)