The jailing of China's former security chief Zhou Yongkang marks the highest point of a corruption crackdown pushed by President Xi Jinping, but efforts appear to be slowing with analysts citing opposition from officials.

Sentenced to life in prison on Thursday, Zhou was once arguably China's third most powerful man, in charge of the country's police, courts and secret service when he retired in 2012.

The prosecution of such a figure shows Xi has consolidated a formidable power base in the ruling Communist party since he became its head in the same year.

But an apparent slowing in the pace of graft probes aimed at senior officials labelled by Xi as "tigers" suggests internal opposition is slowing his high-profile campaign against graft, analysts said.

As a former member of the party's elite Politburo Standing Committee, Zhou is the most senior former official to be jailed for corruption since the Communist party took power almost 70 years ago.

"There was previously a tacit agreement not to tackle anyone on the standing committee," Zhang Ming, politics professor at Beijing's Renmin University said.

"That agreement has now been broken."

The Communist party tightly controls China's court system, determining verdicts in major trials through an opaque process of backroom negotiation.

Analysts said Xi's power is constrained by retired officials such as former President Jiang Zemin, who are eager to protect their reputations and allies in the party.

Joseph Fewsmith, Chinese politics professor at Boston University, said Zhou's downfall "marks a major milestone in Xi's consolidation of power".

But he added: "(party) elders would have had to agree with the verdict".

China's Communist party has around 86 million members, and its internal disciplinary body said 232,000 were punished for graft and other reasons in 2014, up 30 percent from the previous year.

But the vast majority were at lower-levels of government.

- Tigers to foxes -

Rumours of lavish lifestyles and massive corruption of former top-level politicians persist in China.

Strict controls on media mean domestic journalists are effectively barred from publishing independent investigations into senior officials.

An investigation last year by the US-based International Consortium of Investigative Journalists said that relatives of Xi and former premier Wen Jiabao used offshore tax havens to hide their wealth.

In 2012, the New York Times and Bloomberg News published investigations into vast riches said to have been amassed by family members of Wen and Xi.

China said the New York Times report had "ulterior motives".

Though nearly a year has passed since the party said it was probing Zhou, it has not since announced the investigation of any figures close to his level.

State media have focused on a "fox hunt", as the party has labelled its attempts to return to China officials who have fled abroad with allegedly ill-gotten gains.

But the names of the "foxes" identified contain no former high-fliers, and Australian media reported that a local bus driver accused of low-level bribery was among those targeted.

Willy Lam, a political analyst at the Chinese University of Hong Kong, said Zhou's jailing represents "a kind of compromise" with other powerful figures in the party.

"Xi Jinping did not want to alienate too many people," he said, adding: "His anti-corruption campaign has made him many enemies".

Zhang Lifan, an independent political commentator in Beijing, told AFP that the slower pace of high-level corruption probes "gives people the feeling that upper echelons in the party are forming a powerful opposition to the campaign".

"We are dealing with a corrupt system, so any anti-corruption efforts will clearly impact the interests of senior officials," he added.

"With regards to the 'tiger hunt', they cannot continue offending powers within the party".

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New York Times