BEIJING (REUTERS) - Chinese investigators have questioned more than a dozen senior officials, including the country's top prosecutor, about links to retired domestic security chief Zhou Yongkang, who is the subject of a corruption probe, sources said.

Mr Zhou, 71, is the most senior Chinese politician to be ensnared in a graft scandal since the Communist Party swept to power in 1949.

Three sources, with ties to the leadership, said more than a dozen officials of ranks equivalent to a cabinet minister or vice minister have been questioned since last year in connection with the case.

The questioning will not necessarily lead to prosecution but these investigations into the affairs of China's top leaders are unprecedented and highlight President Xi Jinping's determination to crack down on graft.

Mr Xi has warned that corruption, a major source of discontent among China's people, is a threat to the party's survival.

While the probe does not risk a split in the party, as Mr Zhou does not have a popular support base, he still has many loyalists in the country's security apparatus who benefited from his patronage.

The officials who were questioned have not been taken into custody, but are barred from leaving the country, said the sources, who requested anonymity to avoid repercussions for discussing secretive elite politics.

"The special task force is trying to determine if the officials broke the law or breached (party) discipline when they did Zhou Yongkang's bidding," one source told Reuters.

The officials include the country's top prosecutor Cao Jianming and people in the security apparatus - the police force and civilian intelligence agency - that Mr Zhou used to head, they said.

Several lower-ranking Beijing city government officials as well as anchors of state broadcaster CCTV have also been questioned, the sources said.

Mr Zhou was a member of the party's Politburo Standing Committee - the apex of power in the country - and held the immensely powerful post of security tsar until he retired in 2012.

The government has yet to make any kind of official statement about Mr Zhou or the case against him and it has not been possible to contact him for comment.

The cabinet spokesman's office, which doubles as the party's public affairs office, the party's anti-corruption watchdog, the Ministry of Public Security, the prosecutor's office, the Beijing city government and CCTV all did not respond to requests for comment.

DRASTIC MEDICINE

Mr Xi told the party's anti-corruption watchdog on Tuesday that the fight against graft was grim and complicated and that it had to be solved quickly with "drastic medicine".

China has hailed the crackdown on corruption, saying "new progress and achievements" had led to a 13.3 per cent increase in the number of people punished last year. But critics argue that lasting results are unlikely without meaningful political reform.

Several of Mr Zhou's men have been felled, including Mr Jiang Jiemin, briefly top regulator of state-owned enterprises, and former Vice Minister of Public Security Li Dongsheng.

Mr Zhou himself has been put under virtual house arrest at a villa in the northern port city of Tianjin, two of the sources said, adding that his bodyguards, secretaries and driver have been removed.

He is being guarded by soldiers from the elite 38th Army and cannot receive guests, leave the villa or make phone calls without prior approval, the sources said, a sign the leadership is nervous about his lingering influence .

"Contact with the outside world has been totally cut off," one of the sources said.

While the Defence Ministry did not respond to a request for comment on Mr Zhou, the official People's Liberation Army Daily reminded soldiers last month of their duty to follow the party's orders.

"Ensure that the barrel of the gun is firmly placed in the hands of people who are loyal to the party, ensure that the military's thinking, politics and actions are always in step with the party's centre, and resolutely uphold the authority of Central Military Commission Chairman Xi," the daily said.

Mr Zhou's wife, one-time television reporter Jia Xiaoye, and his eldest son from a previous marriage Zhou Bin are also under investigation, the sources said. Mr Zhou's second son Zhou Han has been spared.

"The special task force is trying to find out who tipped off Zhou Bin about the issuance of a warrant for his arrest last year," another source said.

Mr Zhou Bin fled to the United States earlier last year and then went to Singapore, but returned to China after negotiations with Chinese authorities, the source added.

It was also not possible to reach any Zhou family members for comment. It is not clear if any of them have lawyers.

During his five-year watch as security chief, Mr Zhou Yongkang oversaw the police force, civilian intelligence apparatus, paramilitary police, judges and prosecutors. Government spending on domestic security exceeded the defence budget.

But Mr Zhou became too powerful and that position was downgraded during a sweeping leadership reshuffle in 2012.

DEAL TRANSPARENTLY

Mr Xi ordered the special task force formed late last year to look into accusations brought against Mr Zhou by political rivals, sources with leadership ties have said.

In ordering the investigation, Mr Xi broke with an unwritten understanding that members of the omnipotent Standing Committee will not be investigated after retirement.

It is unclear if the government will actually put Zhou on trial and risk the possibility that embarrassing revelations about China's elite become public knowledge, potentially further affecting public faith in the party.

"The party needs to deal with this transparently or risk undermining confidence in its fight against corruption," said a source with ties to the military.

Mr Zhou was last seen at an alumni celebration at the China University of Petroleum on Oct. 1.

Mr Zhou himself was a patron of the once high-flying politician Bo Xilai, who was jailed for life in September for corruption and abuse of power - the worst political scandal since the 1976 downfall of the Gang of Four.

Bo's career was stopped short last year by the attempted defection of his estranged police chief who implicated Bo's wife in the murder of a British businessman over a business dispute.

Bo's wife and his former police chief have been convicted and jailed.