BEIJING (REUTERS) - Chinese President Xi Jinping has pledged to strike hard against graft in the military, urging soldiers to banish corrupt practices and ensure their loyalty to the ruling Communist Party, state media reported on Friday.

The vow to punish graft in the military came only days after the Communist Party began an investigation into former domestic security chief Zhou Yongkang, by far the highest-profile figure caught up in Xi's corruption crackdown.

Xi said troops should remember where their priorities lie, the official PLA Daily reported. His remarks were made during a visit to a military base in the southeastern province of Fujian on Thursday to mark the 87th birthday of the People's Liberation Army (PLA).

"Resolutely punish corruption, maintain the good image of the people's armed forces from beginning to end," the PLA Daily quoted Xi as saying.

He said soldiers must stay clear of what the party calls the "four customs" - formalism, bureaucracy, hedonism and extravagance.

In June, the party announced that it will court-martial Xu Caihou, one of its most senior former military officers, on charges of corruption.

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

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

Xi has made weeding out corruption in the military a top goal. It comes as Xi steps up efforts to modernise forces that are projecting power across the disputed waters of the East and South China Seas, although it has not fought a war in decades.

He stressed the importance of the army's loyalty to the party, the PLA Daily reported, saying "the party's absolute leadership over the army should be unswervingly adhered to".

Xi also called for the military to conduct more drills to improve fighting abilities and to ensure they can win battles, the report said.

Fujian is one of China's most important military locations as it lies opposite self-ruled Taiwan, which the Communist Party considers a rebel province eventually to be brought under Beijing's control, by force if necessary.

China stepped up a crackdown on rampant corruption in the military in the late 1990s, banning the PLA from engaging in business. However, the military has conducted commercial dealings in recent years due to a lack of checks and balances, sources say.