Cambodia's appeals court has rejected a request for the release on bail of opposition leader Kem Sokha, who has been charged with treason in a case that is seen as a partisan political effort by the government ahead of next year's general election.

Lawyers for Kem Sokha, head of the Cambodia National Rescue Party, boycotted the Tuesday hearing when their demand that their client be allowed to attend was denied.

Lawyer Sam Sokong cited the judge saying Kem Sokha was not brought to appear because of security concerns, a reference to the prospect of a protest by the opposition leader's supporters.

Kem Sokha was charged with treason for allegedly conspiring with the United States to topple the government, and could face up to 30 years in prison if convicted.