A delegation met the country's Interior Minister, Sar Kheng, to urge the authorities to free the men unless serious evidence can be shown against them.

Khieu Samphan enjoyed a recent holiday in Sihanoukville

The activists are being held in jail as two senior architects of the Khmer Rouge, under whose regime more than 1.7 million people died, enjoy VIP treatment in the country.

Khieu Samphan and Nuon Chea have been allowed to travel freely around Cambodia and treated to government hospitality following their defections on Christmas Day.

Our Cambodia correspondent Caroline Gluck says the bitter irony is not lost on many Cambodians who say the two cases underscore the flawed nature of the country's justice system.

Five dead

The two human rights workers were arrested after a demonstration against the dumping of 3,000 tonnes of industrial waste by Taiwan's Formosa Plastics Corp in southern Cambodia.

News of the waste sparked riots in the port town of Sihanoukville in which one person was killed. Four others died in a panicked exodus of more than 10,000 residents fearing contamination.

The jailed human rights workers have been refused bail and could be held for up to six months pending their trial.

They hotly dispute the charges against them of robbery and wrongful destruction of property, saying they were monitoring the demonstration.

Anti-Khmer demo ditched

Human rights groups are becoming increasingly concerned about the safety of their staff and cautious in their work.

A coalition of 17 groups, known as The Human Rights Action Committee, has considered holding a peaceful demonstration calling on the government to initiate efforts to bring the two former Khmer Rouge leaders to trial for crimes against humanity.

Taiwan has said it will remove the waste

The idea was dropped because many feared the meeting could be infiltrated by opponents, turn violent and might lead to the organisers facing legal action.

Many human rights groups say unless the perpetrators of genocidal acts during the Khmer Rouge reign of terror are held accountable, impunity from punishment for serious crimes will remain the norm in Cambodia.

Kek Galabru, president of human rights group, Licadho, whose two workers are currently in prison, said it was a very difficult time for organisations like hers.

She said her staff had tried to help suspected victims of the dumped waste, but became victims themselves.

Formosa Plastics Corp said at the weekend it planned to remove the industrial waste which was shipped to Cambodia in late November.