In a statement issued on Friday, one of the groups, Human Rights Watch Asia, said the main obstacle was the determination of second prime minister Hun Sen's Cambodian People's Party to control the process and restrict basic freedoms.

Meanwhile, the two most senior members of Cambodia's opposition parties have said they might not take part in elections scheduled for July 26, unless urgent steps are taken to make them fairer.

In a joint statement, Prince Norodom Ranariddh and his ally, Sam Rainsy, said all hope of free and fair voting had almost disappeared. They said that if there were no improvements they would withdraw from the election by July 5.

Francesc Vandrell (UN Asia-Pacific Political Affairs Director) interviewed on BBC World

In a meeting earlier this week, with the senior United Nations political affairs official, Francesc Vendrell, Hun Sen, repeated assurances that the elections would be free and fair.

A report issued earlier by the International Crisis Group (the ICG) which is chaired by the former US senator, George Mitchell, has recommended that donors insist the polls be delayed until October.

The ICG says there is a growing risk that the whole exercise will become a disreputable farce and warns that if political groups lose faith in the democratic process in Cambodia, the chances of a return to civil conflict will increase.

Cambodia's second PM Hun Sen refuses to delay the polls

Continuing political intimidation, impunity, and lack of access for the media are the main problems highlighted in the reports. But second prime minister Hun Sen has repeatedly brushed-off such complaints and refused to consider any postponement of the polls.

The BBC correspondent in Phnom Penh says that these latest reports echo a series of complaints by opposition parties and local human rights groups who say that the electoral process in Cambodia has been deeply flawed and biased.