A Chinese court has convicted six officials in the drowning of a man reportedly stripped and held under water to try to extract a corruption confession, a lawyer for the victim's family said Tuesday.

The rare case has offered an unusual glimpse into the internal workings of the ruling Communist Party's high-profile anti-corruption drive, although the verdict and sentence have gone unreported by China's state media.

The party maintains its own internal justice system, separate from Chinese state law, run by its feared and secretive Central Commission for Discipline Inspection, for which five of the men worked as corruption investigators.

The sixth man was a local prosecutor.

The group were sentenced late last month by a court in Zhejiang province, according to Wu Pengbin, a lawyer for the family of victim Yu Qiyi.

"On September 30, the court handed down individual sentences of four to 14 years for the six men," Wu told AFP.

The victim, Yu, had been the chief engineer of a state-owned company in the city of Wenzhou. He was arrested in March and detained for more than a month over suspected wrongdoings in a land deal.

He died in April while in the investigators' custody, reportedly after he had been stripped naked and held down in a bathtub as the officials sought to obtain a confession to corruption.

Family members also said they had found multiple bruises on Yu's body.

Yu's detention and brutal treatment at the hands of his investigators bear the hallmarks of "shuanggui" or "two rules", the Communist party's internal interrogation process.

Under it, officials suspected of wrongdoing are ordered to report to a specified location and given a time limit by which they must confess. If they do not do so, increasingly harsh methods will be applied.

They can be held for up to six months with no contact with lawyers or their families, and detainees are often confined in basements to prevent suicide attempts by defenestration, according to rare witness testimony.

Chinese President Xi Jinping took office in March with a pledge to crack down on official corruption at all levels of government, be it among members of the local, provincial or central party.

Graft causes widespread anger among ordinary Chinese, and Xi has described it as a threat to the party.

The anti-corruption campaign has already swept up some high-ranking officials, including Jiang Jiemin, who once oversaw state-owned firms, and Liu Tienan, previously the deputy director of China's National Development and Reform Commission.

The investigators in the Yu case had been accused of intentional injury and were tried last month in Quzhou in Zhejiang.

But one of the family's lawyers later said the relatives' representatives were denied the right to ask questions and were blocked from attending part of the proceedings.

Another of the family's lawyers, Si Weijiang, told AFP all six accused were appealing the ruling. "They don't accept the charge and the sentence," he said.

Yu's family and their attorneys have also called for more senior party officials to be held responsible after one of the defendants reportedly testified that a more senior official had taken the decision to hold Yu under water.

But analysts expressed scepticism that the case would be expanded to ensnare any higher-ranking personnel.

Willy Lam, a Chinese politics specialist at the Chinese University of Hong Kong, said that the government's tight control over the judicial system, combined with the lack of coverage of the incident in the state-run press, made any further repercussions highly unlikely.

"The judicial system is still heavily under the control of the party, and if it is indeed true that somebody higher up should have been penalised, there's kind of a mutual-protection convention," Lam said.

"They will penalise the lower levels of officials, but for mid- to senior-ranked officials, I think that there is a sort of unwritten convention that they won't be touched," he added.

The verdict has not been publicised in Chinese media and barely figured on the country's popular Twitter-like microblogs Tuesday.