PHNOM PENH (AFP) - Human Rights Watch (HEW) on Saturday (Nov 14) urged Cambodian authorities to drop the "politically motivated" case against opposition leader Sam Rainsy as tensions between him and strongman Prime Minister Hun Sen spike.

A court issued an arrest warrant for Mr Rainsy on Friday over an unserved defamation sentence from 2011, a day after Mr Hun Sen threatened him with legal action for comments urging the premier to move towards a peaceful exit from office.

Mr Hun Sen - who has ruled Cambodia for more than three decades - is routinely accused of stifling the opposition and Rainsy has accused his ruling Cambodian People's Party of stealing the last election in 2013 with widespread vote-rigging.

HRW on Saturday called on the authorities to "rescind the politically motivated arrest warrant".

"Cambodia's donors should publicly call for the case to be dropped and for Prime Minister Hun Sen to end his repeated use of the criminal law against political opponents," it said in a statement.

Mr Rainsy, currently on tour in South Korea, was sentenced for defamation in 2011 for accusing the kingdom's foreign minister of being a former member of the brutal Khmer Rouge, which ruled Cambodia from 1975 to 1979.

The opposition chief is Mr Hun Sen's main rival and was in self-exile at the time of the sentence as he also faced a string of other convictions he claimed were trumped-up.

Mr Rainsy's recent comments urging the international community to pressure Mr Hun Sen for full democratisation were made during his trip to Japan earlier this week as he took inspiration from the historic vote in nearby Myanmar.

In a Facebook post on Saturday he again referred to the former junta-ruled state, which has become an unlikely beacon for democracy among its South-east Asian neighbours.

Opposition leader Aung San Suu Kyi's "resounding" victory "has created panic among the last surviving dictators in our part of the world", he wrote.

"But the wind of freedom that is blowing throughout the world will also reach Cambodia in the very near future," said Mr Rainsy.

Neighbouring Thailand has been under military rule since a May 2014 coup, with elections ruled out until at least July 2017, while Vietnam and Laos are ruled by authoritarian communist regimes.

Mr Rainsy only returned to Cambodia ahead of the flawed 2013 elections after a royal pardon for the sentences against him.

According to HRW, the 2011 defamation case was not mentioned in the pardon.