BEIJING (Reuters) - A close ally of President Xi Jinping is expected to be promoted to vice chairman of China s Central Military Commission, four sources said, as Xi cements his control over the armed forces. Zhang Youxia, 67, one of just a few senior military officers with combat experience, is tipped to become one of at least two vice chairmen of the commission, the sources, including three with direct ties to the leadership, told Reuters. Xi is chairman of the commission, which has overall control of the People s Liberation Army, as well as the nation s president and general secretary of the Communist Party of China. Zhang would replace Fan Changlong, who is expected to retire during the 19th Communist Party Congress which begins on Wednesday. The other vice chairman, Xu Qiliang, is expected to stay on, the sources said. Both Xi and Zhang are from the northwestern province of Shaanxi and both are children of former senior officials who fought together in the civil war in the 1940s. China s Defence Ministry did not respond to a faxed request for comment. The party has been studying a proposal to increase the number of vice chairmen on the military commission from two to four and reduce the number of other committee members, currently eight, some of the sources said. Xi has shaken up the commission and military since assuming power late in 2012 as he roots out corruption and streamlines the 2 million-strong armed forces, the world s largest. Some 300,000 troops have been laid off and advanced new equipment such as stealth fighters has been developed. While China has not fought a war in decades, it is taking an increasingly assertive line in the disputed East China Sea and South China Sea, as well as over self-ruled Taiwan, which is claimed by China. Two outgoing members of the commission are currently under investigation for suspected graft, sources have told Reuters, though the government has yet to confirm this.  It isn t the first time the military has faced corruption issues. A previous vice chairman, Guo Boxiong, was jailed for life for graft last year, while another former vice chairman, Xu Caihou, died of cancer in 2015 before he could face trial over alleged corruption. Zhang is currently the eighth-ranked member of the 11-man military commission. Zhang is also expected to be promoted to the Communist Party s 25-member Politburo, one of its elite ruling bodies, three of the sources said. One source with ties to the leadership said that the fathers of Xi and Zhang - Xi Zhongxun and Zhang Zongxun - had been close.  The two fathers fought together in the civil war that ended in 1949 with the Communist victory and proclamation of the People s Republic of China. Both men rose to senior positions in the government and military.  The second generation of the two families are also close,  the source added, referring to Xi and Zhang. Born in Beijing, Zhang Youxia joined the army in 1968, rising through the ranks and joining the military commission in late 2012.  He fought against Vietnam in a brief border war in 1979 that China launched in punishment for Vietnam invading Cambodia the previous year and ousting the Beijing-backed Khmer Rouge. Zhang was 26 when he was sent to the frontlines to fight the Vietnamese, where he performed well and was quickly promoted, according to state media. He also fought in another border clash with Vietnam in 1984.  During the battle, whether attacking or defending, Zhang Youxia performed excellently,  the official China Youth Daily wrote in August in a piece entitled,  These Chinese generals have killed the enemy on the battlefield . Another officer recently promoted, Li Zuocheng, who was named chief of the Joint Staff Department of the People s Liberation Army in August, also fought in the 1979 war. Li is also likely to become one of the commission s vice chairmen, but it was unclear if he will also join the Politburo, the sources said. One of the sources, who has ties to the military, said Zhang had a reputation when he was based in northeastern China of walking around his base dressed in civilian clothes, prompting challenges from junior soldiers who didn t recognize him.  He doesn t put on airs and likes getting down in the dirt with ordinary soldiers, to know what their lives are like, how they are living and what they are eating,  said the source, speaking on condition of anonymity because of the sensitivity of the matter. 