"Piles and piles of bodies" was the way the UN described the scene in the South Sudanese city of Bentiu. The government claims 400 civilians were ethnically cleansed by rebels allied with former Vice President Riek Machar.

Indeed, according to this report in the Washington Post, the rebels made no secret of their desire to massacre people not of their ethnic background:

Born in 2011, South Sudan came to be with the help of the United States out of the rubble of a Sudanese civil war. A split between President Salva Kiir and rebel leader Riek Machar triggered fighting between their forces in December. Since then, an estimated 1 million people have been driven from their homes, about 800,000 of them internally displaced and another 200,000 refugees driven into into Kenya, Uganda, Ethiopia and Sudan.

But these killings were, in some ways, different from those that have come before. The rebels made no secret of their plans. A local radio station featured rebel commanders warning certain ethnic groups, everybody but the Nuers, that they were coming for them, calling on the other groups to rape the non-Nuer women.

People had sought refuge in places of worship and healing. But as has been the case in other towns, there wasn’t any after the killing began on April 15.

The killers, identified by the United Nations as forces of the Nuer-led “Sudan People’s Liberation Movement in Opposition Army” led by former vice president Machar, went from place to place, from mosque to church to hospital, separating people by ethnicity and religion and shooting the ones left behind.

The people spared were mostly from the Nuer community. But not all of them made it. Some Nuers who hid were shot, too, according to reports. “Several Nuer men, women and children were killed for hiding and declining to join other Nuers who had gone out to cheer” the rebel forces, said the U.N. report after the killings.

“It’s the first time [in South Sudan] we’re aware of that a local radio station was broadcasting hate messages encouraging people to engage in atrocities,”  the United Nations’ Toby Lanzer, who was in Bentiu on Sunday and Monday, told the AP by phone. “And that really accelerates South Sudan’s descent into an even more difficult situation from which it needs to extract itself.”

Observers on the ground say it may get a lot worse.

Lanzer reported that thousands of civilians from several ethnic groups streamed to the U.N. peacekeeping base in Bentiu because many believe more violence is coming. “The base now holds 22,000 people — up from 4,500 at the start of April — but can supply only one liter of water per person per day. Some 350 people must share one toilet.”

“The risk of a public health crisis inside our base is enormous,” he said.

About all the rest of the world can do - short of sending in troops to physically protect the population - is make witness to the disaster. Since that will never happen, the rebels will continue their efforts to ethnically cleanse the oil-rich region and seize it for themselves.