HARBIN, July 9 -- A Chinese convict has been given another 10 years in prison for seducing and blackmailing a number of women while behind bars in northeast China's Heilongjiang Province.

Wang Dong will serve a further 13 years and nine months, including the time remaining under his original six-year sentence for kidnapping, the Nehe municipal court said Thursday. He has been in Nehe Prison since December 2012.

Wang, 28, used cellphone app WeChat to chat with several women living near the prison last year and "established lover relationships with them".

He asked two women to visit him in prison, where he extorted over 110,000 yuan (about 18,000 U.S. dollars) from them by threatening to spread videos and pictures that they had sent of themselves naked. One female visitor surnamed He, whose husband is a police officer, alleged that she had sex with Wang in prison. Wang denies this.

The blackmail came to light when a third woman threatened by Wang turned to the police in late 2014.

Many have deemed the case unbelievable as cellphones are banned in prisons and visits are monitored by prison guards. The case has caused a public outcry about lax supervision and corruption in Chinese prisons.

Wang owned five mobile phones, three of which were sent by a female victim with the help of prison guards after they took bribes, according to investigators.

In January, the warden and political commissar of the Nehe Prison were dismissed. Three guards are being investigated by prosecutors for suspected dereliction of duty and abuse of power.

Wang pled guilty and did not appeal after the trial, which was closed for personal privacy reasons, the court said.