A UN-backed Cambodian tribunal has begun hearing the first genocide case against the country's 1970s Khmer Rouge regime.

Khieu Samphan, the regime's head of state, and Nuon Chea, right-hand man to the goup's late leader, Pol Pot, have already received life sentences in August after being found guilty of charges including crimes against humanity.

They are now facing separate charges of genocide related mostly to the group's forced movement of millions of people to the countryside when it took power in 1975.

The radical policies are blamed for the deaths of an estimated 1.7 million Cambodians from starvation, exhaustion, disease and execution. Both men have appealed their convictions.

The trial, which started in July, reopened on Friday with an opening statement by the prosecutors, and an opportunity for the accused to respond.

Deputy co-prosecutor William Smith said the hearings "will ensure a more comprehensive accounting" of the crimes of which the ex-leaders are accused, so that "Cambodia's past is not buried but built and learnt from".

Prosecutors are expected to call upon their first witness on Monday.

The complex case against the pair was split into a series of smaller trials in 2011 to get a faster verdict given the vast number of accusations and the advanced age of the accused.

Mass killings

The mass killings of an estimated 100,000 to 500,000 ethnic Cham Muslims and 20,000 Vietnamese form the basis of the genocide charges against Khieu and Nuon.

Before these charges were filed, the treatment of the minority Muslim group and Vietnamese community was rarely discussed.

"The ways in which the Khmer Rouge mistreated us is too heinous to describe in words. Their goal was to exterminate our race," said Seth Maly, a 64-year-old Cham labour camp survivor who lost 100 of her relatives, mostly through execution, during the regime - including her two daughters, parents and five siblings.

Nuon and Khieu also face charges of crimes against humanity and war crimes in the second trial - for the deaths of up to two million people through starvation, overwork or execution during the communist regime.

Most of these deaths do not fall under the charge of genocide, which is defined by the United Nations as "acts committed with intent to destroy, in whole or in part, a national, ethnic, racial or religious group".

"Without a second trial, there would be an enormous gap in the legal record about crimes that defined the experience of - and still traumatise - regime survivors," said Anne Heindel, a legal adviser to the Documentation Center of Cambodia, which researches the country's bloody history.

Led by "Brother Number One" Pol Pot, who died in 1998, the Khmer Rouge dismantled Cambodian society in a bid to create an agrarian utopia.

The hearings will also provide the first forum for justice for tens of thousands of husbands and wives forced to marry, often in mass ceremonies, as part of a Khmer Rouge plan to boost the population.

A court spokesman has estimated the trial may go on until 2016, with hearings covering crimes committed at Khmer Rouge labour camps and prisons including the notorious Tuol Sleng, also known as S-21.

Source: Agencies