Former Khmer Rouge leaders on trial for crimes against humanity "spilt blood for power" in their quest to make the country a slave state, prosecutors alleged on Friday.

"Brother Number Two" Nuon Chea, 87, and ex-head of state Khieu Samphan, 82, are accused of playing a critical role in the communist regime which wiped out a quarter of the population in the late 1970s.

"Nuon Chea and Khieu Samphan killed for power. They spilt blood for it. They brutalised and dehumanised their own people and kept spilling blood for power," prosecutor William Smith said in closing statements at Cambodia's UN-backed war crimes court.

But he said the two leaders ordered others to do "their very dirty work" for them.

"Seeing your victim's eyes makes it hard to kill," Smith said.

"If you look close enough you see your own humanity in their eyes. Nuon Chea and Khieu Samphan chose not to see the eyes of their victims. They chose not to see their own humanity."

More than three decades after the country's "Killing Fields" era, the UN-backed court is moving closer to a verdict in their complex case, which has been split into a series of smaller trials.

The first trial has focused on the forced evacuation of people into rural labour camps and the related charges of crimes against humanity.

The evacuation of Phnom Penh in April 1975 was one of the largest forced migrations in modern history.

More than two million people were expelled from the capital at gunpoint and marched to rural labour camps as part of the regime's plan to forge an agrarian utopia.

Smith said Nuon Chea and Khieu Samphan were prepared to do whatever it took to realise their radical vision of Cambodia as "a slave state".

"Nuon Chea and Khieu Samphan and their criminal partners were the masters and the Cambodian population were their slaves.

"They were dictators who controlled Cambodians by brutal force and fear," he added

Closing statements in the trial are scheduled to be completed by the end of the month, with a verdict expected in the first half of next year.

The defendants deny charges of war crimes, crimes against humanity and genocide.

Other allegations, including genocide and war crimes, are due to be heard in later hearings although no date has yet been set.

Led by Pol Pot, who died in 1998, the communist regime wiped out up to two million people -- nearly a quarter of the population -- through starvation, overwork and execution between 1975-79.