Xi, the ruling Communist party's General Secretary and the Central Military Commission's Chairman, made the comments while inspecting the Southern Theatre Command of the People's Liberation Army (PLA) on Friday and stressed building a strong army which should also avoid being corrupt.
Xi, 63, viewed as the most powerful leader heading the party, military and the government will complete his first five year term this year and expected to be re-elected for another five-year term during the 19th Party Congress to be held later this year.
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He vowed to fight corruption and enhance the "sense of gain" among officers and soldiers of the 2.3 million-strong People's Liberation Army, the world's largest.
Xi asked military personnel to strengthen their awareness in preparing for war, closely follow changes of situations and make unremitting efforts to enhance combat capabilities.
Xi required an accelerated building of the theatre joint combat command system, vigorous development of a new-type fighting force and simultaneous improvement of national defence strength and economic development, the official Xinhua news agency quoted him as saying.
His comments came amid China's maritime disputes with its South China Sea neighbours.
China claims almost all of the South China Sea, including islands more than 800 miles from the Chinese mainland, despite objections from neighbours such as the Philippines, Malaysia, Brunei and Vietnam.
Beijing has also created artificial islands in the area, outfitting some of them with military features. The issue is a source of ongoing tension and anger in the region.
In his speech, Xi urged all military personnel to resolutely safeguard the authority of CPC Central Committee, unswervingly follow the Party's leadership and combat corruption.
He asked all military personnel to greet the 19th CPC National Congress scheduled for later this year with "outstanding achievements," Xinhua quoted him as saying.
Noting that 2017 is of great significance for the Party and the country, Xi urged the PLA to strengthen ideological building, combat preparation and reform implementation.
Xi urged PLA officers to eliminate the impact of Gen Guo Boxiong and Gen Xu Caihou, two corrupt former CMC vice chairmen, and strictly observe political discipline and rules.
"It must be ensured that the PLA resolutely follows the command of the CPC Central Committee and the CMC at any time, in any circumstances," Xi said.
A campaign, which focuses on the study of the Party Constitution and rules, as well as the speeches by Xi, and calls for being qualified Party members, is required to be further launched in the army.