By Benjamin Kang Lim and Ben Blanchard

BEIJING (Reuters) - China's former domestic security chief Zhou Yongkang, one of the country's most powerful politicians of the last decade, is helping authorities in a corruption probe and, contrary to media reports, is not currently the target of the investigation, sources told Reuters.

The investigation could take weeks, maybe months, to complete. Even if Zhou is implicated, he is unlikely to follow in the footsteps of disgraced ally Bo Xilai and face prosecution, said the sources, who have ties to the leadership or direct knowledge of the matter.

The scandal involving Bo, who is awaiting the verdict of a trial that concluded last week on charges of corruption and abuse of power, and the rumors surrounding Zhou are the most serious convulsions within the ruling Communist Party in decades. The probe Zhou is helping in has already reached deep into China's state-owned oil companies and several senior executives are being investigated for graft.

President Xi Jinping, who formally took power in March, has vowed strong action against corruption, but he may not yet be powerful enough to take on Zhou, who still wields political clout through protégés he had promoted into key positions.

Any action against Zhou may only be considered after the party's elite 200-member Central Committee meets at a plenum in November, the sources said. Xi will need unstinted support at the meeting, where he is likely to present wide-ranging reforms to rebalance the economy.

"Zhou has not been 'shuang gui'," one source told Reuters, referring to a form of internal investigation in which a suspect is required to confess within a prescribed time and place.

"He was merely asked to assist with the corruption investigation," the source said. All three sources declined to elaborate when asked if the probe involved Zhou's family members or allies at state oil giant China National Petroleum Corp, of which he was president in the 1990s.

"Unlike Bo, Zhou is unlikely to be arrested or put on trial" even if he is implicated in the probe, the source added.

Media reports that Zhou was being investigated came after inquiries into at least eight allies linked to him, including the country's top regulator of major state-owned enterprises who has been sacked for an unspecified "serious breach of discipline".

Hong Kong's South China Morning Post reported last week that Zhou was facing a corruption probe. U.S.-based China-watching news site Duowei said earlier Zhou was under investigation for graft but later withdrew the report for unknown reasons.

While dismissing the reports, the sources said the investigations of his allies did appear aimed at marginalizing Zhou, 70, who retired as security tsar and from the Communist Party's all-powerful Politburo Standing Committee during a sweeping leadership reshuffle last November.

"Whether they will touch Zhou Yongkang or not is another matter, but that message is already very clear - it's undermining one of the most important vested interest groups," said Cheng Li, a senior fellow at the Brookings Institution in Washington.

UNWRITTEN RULE SCRAPPED

In a landmark move earlier this year, the Communist Party scrapped a decades-old unwritten rule that exempts incumbent and retired Standing Committee members from investigation for corruption, the sources said.

The party can now open an investigation into purported evidence provided by a named - not anonymous - whistleblower against a sitting or former Standing Committee member, the sources said, requesting anonymity to avoid repercussions for discussing secretive and sensitive elite politics.

But any decision by Xi to proceed to the next step - prosecution - still needs the approval of the incumbent Standing Committee, the sources said, adding that senior retired leaders would also need to be consulted.

"Xi cannot unilaterally decide to arrest Zhou. It would need the approval of the Standing Committee as well as party elders," a second source said.

Any move against Zhou would be unprecedented since no sitting or retired Standing Committee member has been jailed for economic crimes since the Communists swept to power in 1949.

"The political risks are too high," the second source said. "Retired Standing Committee members would be smashing their (collective) foot with a rock if they agree to prosecute one of their own. Any one of them could be next."

Zhou could not be reached for comment and the party's anti-corruption watchdog did not answer telephone calls. The State Council Information Office, or cabinet spokesman's office which doubles as the party spokesman's office, did not respond to requests for comment. A Foreign Ministry spokesman said questions on Zhou were outside his remit.

The sources said Zhou's movements have not been restricted, but, in any case, travel outside Beijing by any retired Standing Committee member needs the approval of the incumbent Standing Committee.

As domestic security tsar, Zhou oversaw the nation's police force, paramilitary People's Armed Police, prosecutors, judges and the civilian intelligence apparatus. During his watch, government spending on domestic security exceeded the budgets for defense, health care or education.

"Zhou was ranked ninth in the (previous) Standing Committee, but he wielded power second only to Hu Jintao in the security realm," another source said, referring to the former president, Xi's immediate predecessor.

"If Xi arrests Zhou, it would upset the balance of power at the top," the source said. "Xi sees the political risks. If he makes too many enemies, he could find himself in trouble."

Xi, who is also party and military chief, has pledged to go after "tigers" and "flies" in the battle against corruption, referring to political heavyweights and lightweights.

But Xi is still consolidating power less than a year into the job.

Zhou had recommended that Bo succeed him as domestic security chief before the latter's dramatic downfall last year, multiple sources with direct knowledge of the matter or ties to the leadership have said.

Bo fell from grace after his police chief, Wang Lijun, sought political asylum at a U.S. consulate and implicated Bo's wife, Gu Kailai, in the murder of a British businessman. Wang and Gu have been meted suspended death sentences which are likely to be commuted to life imprisonment.

When Zhou stepped down along with most members of the Standing Committee at the 18th Party congress last November, the role of domestic security tsar was downgraded, reflecting leadership fears that the position had become too powerful.

(Additional reporting by Sui-Lee Wee, and John Ruwitch in SHANGHAI; Editing by Raju Gopalakrishnan)