Nearly half of Cambodia's opposition parliamentarians have fled abroad in the past month to escape a government crackdown that has battered their party with legal cases ahead of crunch polls, an MP said on Wednesday.

The exodus further imperils the prospects of a party that poses the only viable challenge at the ballot box to premier Hun Sen, who has clung on to power for 32 years.

The strongman has smothered dissent in the lead-up to the 2018 national election and is accused of picking off his critics through a series of dubious court cases.  

The crackdown was ramped up last month when opposition leader Kem Sokha was thrown into a remote jail on treason charges, a dramatic arrest that sent fear rippling through the embattled opposition movement.

Since then, more than 20 opposition politicians have fled the country, MP Mao Monyvann said of his Cambodia National Rescue Party (CNRP), which has 55 members of parliament.

"Now there are only a few more than 10 (MPs) still in the country," he told AFP, with others having fled before Kem Sokha's arrest. 

Many have left for the US or European countries where they have family or dual citizenship. 

Mu Sochua, an outspoken deputy to Kem Sokha, was the latest to bolt on Tuesday, a day after Hun Sen said there would be no let-up in his crackdown.

The bellicose premier has ratcheted up his rhetoric in daily speeches to the public, with threats to dissolve the CNRP if they continue to "protect" their leader.

"I don't intend to continue to live under the continuous and open threat by the PM of arrest," Mu Sochua told AFP through Facebook, without sharing her whereabouts. 

Human Rights Watch's Phil Robertson said the departure of MPs could spell the "death of Cambodian democracy" at the hands of Hun Sen.  

"Hun Sen wants to run in a 2018 election in which he will not face any meaningful opposition," said HRW's Deputy Asia Director, adding the poll would not be "free nor fair under these circumstances".

In a message sent through his lawyers on Monday, Kem Sokha denied the treason charge as "total slander", a view echoed by the US and other democratic countries which have called for his immediate release.

Hun Sen loomed over the impoverished democracy for more than three decades, using a mix of political wits and hard power to undermine his rivals and secure the loyalty of the bureaucracy, courts and security forces.  

But his popularity slipped in a 2013 election and he has since moved to silence critics in politics, the media and NGOs.

Around a dozen opposition politicians have faced prosecutions since that poll, with Kem Sokha's predecessor Sam Rainsy forced to flee abroad to avoid a string of convictions which he says are politically motivated.

Last week the UN's human rights envoy to Cambodia warned the country was "moving inexorably towards a precipice".

Source: AFP