Prime Minister Hun Sen is using prisoners as bargaining chips to try and force Cambodia National Rescue Party deputy leader Kem Sokha to sign a declaration that would be used to kick opposition president Sam Rainsy out of the party, the latter claimed yesterday.

Via email, Rainsy said Sokha had been presented with a statement to sign pledging to expel any member of the CNRP who had insulted the premier’s family by alleging his eldest son Hun Manet was fathered by Vietnamese revolutionary general Le Duc Tho.

Rainsy said signing the letter was a condition for releasing four employees from rights group Adhoc and an election official charged with bribing the purported mistress of Sokha, a case widely believed to be politically motivated.

He claimed the CPP had “bought” one or two members of the CNRP, who would be called upon to accuse him of insulting the premier’s family, at which time Sokha would be leaned on to make good on the conditions of the statement.

Rainsy, who said any suggestion he had insulted the premier’s family were “groundless”, did not provide the names of the supposed plants within the opposition.

“Kem Sokha adamantly refuses to write such a tricky letter,” Rainsy said, saying a draft version of a statement had been sent to Sokha via media mogul Soy Sopheap, who declined to comment yesterday.

“Hun Sen says then the prisoners will remain in jail. He says he will not do anything to help their release the prisoners are just Hun Sen’s hostages who are being used to blackmail the CNRP,” Rainsy added.

The five detainees were arrested in April after being ensnared in an investigation into Sokha’s alleged affair, which also saw an opposition commune chief jailed and Sokha sentenced to five months in prison for refusing to appear at court in a related case.

Recent royal pardons granted to Sokha and the commune chief raised hopes that the group would also be released.

However, the alleged leveraging of their freedom adds further weight to suggestions by observers that the pardons were part of a ploy to try to split Sokha and Rainsy, who fled abroad in 2015 to avoid arrest and has since been officially barred from the country.

Following his pardon, Sokha left his refuge at the CNRP headquarters, and pledged to secure the release of the remaining prisoners before the end of the year, though they remain in limbo.

Sokha is set to soon meet Interior Minister Sar Kheng and they are expected to discuss the matter. According to a letter exchange, Kheng has agreed to meet, but has not yet set a date, citing a busy schedule.

Reached yesterday, CPP spokesman Sok Eysan said the case of those imprisoned for bribing Sokha’s mistress was “in the hands of the court” and the party couldn’t intervene.

As for the demand that Sokha expel CNRP members who insulted the premier’s family being a condition of their release, Eysan said it was simply an invention of Rainsy’s.

“Sam Rainsy is good at using tricks,” he said.

A purported version of the draft letter for Sokha to sign included in Rainsy’s email calls any suggestion that the premier’s wife was the mistress of Duc Tho, a Vietnamese revolutionary leader, “unacceptable” and says the party will expel any “ignorant individuals” who make the claims from the party.

It is not the first time rumours about Manet’s birth have caused trouble for the CNRP.

In May, the party terminated the membership of US activist Brady Young after he spread the rumour in online videos to the premier’s chagrin. Rainsy then sent a letter to Hun Sen condemning the claim.

In a public speech yesterday, Hun Sen warned the opposition against criticising his party. “Dishonest and insulting works to attack each other, and incitement which provokes hatred and anger, will not sustain the culture of dialogue and it will destroy out national unity.”