Officers at a prison in Northeast China have denied that one of their prisoners had sex with a policeman's wife while serving a sentence, according to media reports.

Nehe Prison in Nehe, Heilongjiang province, did not submit video records to a team dispatched by the Heilongjiang provincial authority, saying that it could not find them, China Central Television reported on Friday.

A preliminary investigation showed that the prisoner, surnamed Wang, used a mobile phone to chat with a number of women, and two of the women gave Wang mobile phones after paying 200 yuan ($32) to a police officer.

Wang's own mobile phone had been confiscated, the report said.

With the help of some prison officers, Wang was able to meet two women under the ruse of being husband and wife.

Zhou Chao, a reporter at Shanghai-based thepaper.cn, told CCTV that Wang had sex in an office with one of the two women - who in previous reports was identified as the wife of a police officer - after she paid a guard 3,000 yuan.

In November, the police officer filed a report to the prison accusing Wang of threatening and harassing him.

Officers denied Wang had sex in prison, but the investigation team is trying to retrieve video records and will make their findings public, according to the report.

Wang, 28, has been imprisoned three times since 2005, and in September 2012 he was sentenced to six years for kidnapping, thepaper.cn reported.

He built relationships with several mistresses through WeChat while in prison, and swindled 80,000 yuan from a woman living nearby, according to thepaper.cn.

So far, six officers in Nehe Prison have been fired or received warnings, the report said.

"It is easier for prisons to be immune from outside supervision due to their exclusiveness, which can result in irregularities and even judicial corruption," said Cheng Lei, a law professor at Renmin University of China. "It is necessary to put prisons under effective and regular supervision so they operate with improved transparency."

Qiao Zhifeng, a media commentator, said, "The authorities should thoroughly investigate the case, and address the gaps in prison management that the case has exposed".

wangxiaodong@chinadaily.com.cn