A Cambodian court on Monday charged six people -- including a local United Nations employee -- over an alleged attempt to bribe a woman so she would deny an affair with a top opposition party member.

It is the latest twist in a sex scandal that has gripped the country since phone recordings of conversations allegedly between lawmaker Kem Sokha and a 25-year-old hairdresser were leaked two months ago.

Rights groups say the legal proceedings are the latest attempt by Prime Minister Hun Sen to undercut a resurgent opposition trying to end his 31-year grip on power.

The woman initially denied the affair with the MP, who is the deputy leader of the Cambodia National Rescue Party.

But she later told a court it had taken place and accused a prominent advocacy group, the Cambodian Human Rights and Development Association (ADHOC), of instructing her to deny the relationship.

On Thursday the country's anti-corruption unit arrested four members of the rights group along with a former group employee. They remain in custody.

"We have decided to charge them with bribing a witness," Ly Sophanna, spokesman for the prosecutor's office at the Phnom Penh Municipal Court, told AFP.

A UN rights office staff member, Sally Soen, was also charged with being an accomplice to an attempted bribe, he added, although he has not been arrested.

Prosecutors have yet to reveal details of the alleged bribe.

Under the criminal code the group face up to 10 years in prison if convicted.

Khem Sokha has not publicly commented on the accusations against him.

A local administrator was jailed last week pending trial for allegedly bribing the hairdresser's family to pressure her into denying the affair.

In addition prominent political analyst Ou Virak will appear in court next week after being sued by Hun Sen's ruling party for defamation. He had suggested the sex scandal was aimed at damaging the opposition.

Hun Sen's main rival, opposition leader Sam Rainsy, has been forced to live in self-exile abroad to avoid arrest warrants which he says are politically motivated.

In a Facebook post last week, Hun Sen asked his officials to prepare lawyers to sue anyone who slanders his party.