BEIJING (REUTERS) - China's police chief wrote on Friday that his officers must uphold the leadership of the Communist Party and be loyal to it, as the government targets the domestic security apparatus in a crackdown on corruption.

Sources have told Reuters that China's former security tsar, Zhou Yongkang, one of the most powerful politicians of the decade, has been put under effective house arrest while the party investigates corruption allegations against him.

Last month, the government began a graft investigation into a one-time deputy public security minister, Li Dongsheng, an ally of Zhou's.

Li held a rank equivalent to cabinet minister, and state media says he is the first member of the ruling party's Political and Legal Affairs Committee, the influential domestic security body which Zhou used to head, to be investigated for graft.

Writing in the party's official People's Daily, Public Security Minister Guo Shengkun said his more than two million officers had to be "absolutely loyal and absolutely clean" and stand steadfastly in line with the orders and politics of President Xi Jinping.

All public security personnel must "take real actions to resolutely defend the leadership of the Communist Party", Guo said. "Unswervingly be a loyal defender of the party and the people." Guo made no direct mention of Li's case or of Zhou. The government has yet to make any official statement about Zhou.

President Xi has made fighting deeply ingrained graft a central theme of his new administration, and has promised to take down high-level "tigers" as well as lowly "flies".

Zhou had expanded his role into one of the most powerful, and controversial, fiefdoms in the one-party government, and under his stewardship the domestic security budget exceeded that of the military.

But Zhou was implicated in rumours in 2012 that he hesitated in moving against Bo Xilai, who had been a contender for top leadership but fell in a divisive scandal following accusations Bo's wife murdered a British businessman.

When Zhou retired in late 2012, the position he occupied was downgraded, and his successor Meng Jianzhu is only a member of the Politburo, the 25-member body which reports to the elite Politburo Standing Committee.

Guo - whose boss is Meng - also warned his officers to be on guard against the "ideological infiltration of anti-Chinese Western forces", likely a reference to voices in China pushing for political reform, something Xi has shown no sign of allowing.

President Xi confounded expectations he may loosen up upon his appointment as president last year, and has instead overseen a sweeping crackdown on dissent, a theme Guo implied would continue in the interests of national development.

"Only if there is social stability can reform and development continue to proceed," he wrote.