Two former Khmer Rouge leaders began their second trial at a UN-backed court in Cambodia Wednesday on charges including genocide of Vietnamese people and ethnic Muslims, forced marriages and rape.

The complex case of the regime's two most senior surviving leaders has been split into a series of smaller trials, initially focusing on the forced evacuation of people into rural labour camps and related crimes against humanity.

The first trial against "Brother Number Two" Nuon Chea, 88, and former head of state Khieu Samphan, 83, was completed late last year, with the verdict -- and possible sentences -- set to be delivered on August 7.

At the opening hearing of the second trial, judge Nil Nonn read out the charges against both suspects, including genocide and other crimes against humanity, as more than 300 people watched the proceedings from the public gallery.

Nuon Chea did not attend for health reasons, while Khieu Samphan sat in court alongside his defence team. Both men deny all charges.

Anne Heindel, a legal adviser to the Documentation Center of Cambodia which researches the country's bloody history, said the second trial was "immensely important for survivors".

"The subject matter of the first proceeding was quite limited, discussing only a narrow set of events that occurred while or shortly after the Khmer Rouge took power.

"(This new trial) focuses on crimes that occurred after the Khmer Rouge were entrenched and implementing long-standing plans for transforming Cambodian society, yet for which no one has ever been held accountable," she said.

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 Nuon Chea and Khieu Samphan.

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

Kob Tiyum, a 65-year-old Cham survivor of the Khmer Rouge's 1975-1979 regime, said the trial would "recognise our suffering".

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She lost two of her children, her father and her brother who died of starvation while working at a labour camp under the regime.

"The Khmer Rouge killed Chams because they wanted to eliminate the race. They did not allow us to speak the Cham language or to pray," she said after attending the hearing.

- Forced marriages -

Nuon Chea and Khieu Samphan also face a string of other charges for the deaths of up to two million people through starvation, overwork or execution during the 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".

It is not known how long the second trial will last, but court spokesman Lars Olsen estimated it 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.

"This trial is very important for me as a victim who lost both parents in Tuol Sleng," said 45-year-old Norng Chan Phal, one of just a handful of survivors from the prison.

"Those criminals who committed genocide and killed their own people must be punished seriously."

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 new trial 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.

The rape charges refer to rape within the forced marriages.

In its historic debut trial, the court in 2010 sentenced former Tuol Sleng prison chief Kaing Guek Eav to 30 years in prison -- later increased to life on appeal -- for overseeing the deaths of 15,000 people.

But observers and victims have raised concerns that the ageing Khmer Rouge leaders may not survive to see a verdict.

Former regime foreign minister Ieng Sary died aged 87 last year while on trial for war crimes and genocide, while his wife was freed from jail in September 2012 after being ruled unfit for trial because of failing mental health.