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PHNOM PENH (Reuters) - Cambodia’s exiled former opposition leader Sam Rainsy was sentenced in absentia on Thursday to an additional 20 months in prison for defamation, on top of a previous five-year term.

Opponents of Prime Minister Hun Sen accuse him of using defamation cases and other tactics to marginalize rivals ahead of local elections in June and a general election next year.

The case against Sam Rainsy had been brought by Hun Sen himself over a radio interview in which he alleged “state authorities” were behind last year’s killing of a government critic, Kem Ley.

Phnom Penh Municipal Court judge Y Thavrak fined Sam Rainsy $2,500 in addition to the 20-month jail sentence. Sam Rainsy was ordered to pay Hun Sen a symbolic 100 riels ($0.025) in damages.

Sam Rainsy’s lawyer had argued there were no grounds for defamation because he had not referred to anyone by name in the radio interview.

A man was jailed last week after confessing to murdering Kem Ley, but human rights groups and the activist’s family said the case left many questions unanswered.

Sam Rainsy resigned from the leadership of the Cambodia National Rescue Party in February, in the face of a threatened government ban on any political party whose leader has been convicted of an offence.

Sam Rainsy has been convicted on a series of defamation charges and has lived in France since 2015 to avoid them.