One of China's most senior former military officers has confessed to taking "massive" bribes in exchange for help in promotions, state media said on Tuesday, as the government moves closer to his court martial as part of its war on graft.

Xu Caihou retired as vice chairman of the powerful Central Military Commission last year and from the ruling Communist Party's decision-making Politburo in 2012.

President Xi Jinping heads the Central Military Commission, which controls the 2.3 million-strong armed forces, the world's largest, and has repeatedly reminded them to be loyal to the party.