A United Nations peacekeeper stands guard Sunday near the scene where about 200 people were killed during an attack in Bentiu, South Sudan. (Reuters)

Gunmen in South Sudan who targeted civilians, including children and the elderly, left “piles and piles” of bodies, many of them in a mosque and a hospital, the United Nations’s top humanitarian official in the country said Tuesday.

Toby Lanzer told the Associated Press in a phone interview Tuesday that the ethnically targeted killings in a provincial capital are “quite possibly a game-changer” for a conflict that has been raging since mid-December and has exposed long-standing ethnic hostilities.

There was also a disturbing echo of Rwanda, which is marking the 20th anniversary this month of its genocide, in which an estimated 1 million people were killed. In the Rwandan genocide, orders to kill were broadcast by radio — and it happened in South Sudan, too, Lanzer said.

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

U.N. human rights investigators said late Monday that hundreds of civilians were killed last week because of their ethnicity after rebel forces seized Bentiu, the capital of oil-producing Unity state. Those rebel forces are Nuer, the same ethnic group to which former vice president Riek Machar, who is now a rebel leader, belongs.

Lanzer said thousands of civilians from several ethnic groups are streaming to the U.N. peacekeeping base in Bentiu because many think more violence is coming. The base now holds 25,000 people but has only one liter of water per person per day and only one latrine per 350 people.

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

Raphael Gorgeu, head of Doctors Without Borders in South Sudan, said people will die inside the U.N. base in coming days because of the water and sanitation situation.

As rebel forces entered Bentiu last week, residents were led to believe that by entering the mosque they would be safe, Lanzer said, citing accounts from survivors. But once inside, they were robbed of money and cellphones, and a short while later, gunmen began killing, inside both the mosque and the city hospital.

Those who were not Nuer could not be saved, Lanzer said. The gunmen killed wantonly, including children and the elderly, he said.

Gorgeu said his team members in Bentiu — including 12 international staff members — have treated more than 200 people wounded in the violence, many gunshot victims among them.

British Ambassador Ian Hughes said Tuesday that the April 15-16 killings are a clear violation of international law. He said those behind the atrocities and those inciting the killings will be held to account.

The violence is only one part of a dual crisis that South Sudan faces. Because of the fighting, more than 1 million people have fled their homes, and very few residents are tending to their crops. Lanzer said there is a severe risk of famine in coming months because April and May is the period when residents should be planting and cultivating.