A U.S. group which observed Cambodia's election last month said Saturday opposition parties had been deprived of meaningful opportunity to appeal the result.
The National Democratic Institute for International Affairs and another U.S. group, the International Republican Institute, initially gave a positive assessment of the election.
But the NDI said in a statement Saturday it had concerns about the credibility of the two bodies meant to rule on electoral complaints, the National Election Committee and the country's top legal body, the Constitutional Council.
"The NEC and the Constitutional Council lack credibility in their responses to election-related complaints and post-election controversies," the group said.
The opposition, led by deposed co-premier Prince Norodom Ranariddh and former finance minister Sam Rainsy, whose parties came second and third respectively, have alleged widespread fraud by election winner Hun Sen's ruling Cambodian People's Party.