NAIROBI, Kenya >> 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 U.N.’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 that has exposed longstanding ethnic hostilities.

There was also a disturbing echo of Rwanda, which is marking the 20th anniversary this month of its genocide that killed an estimated 1 million people. The Rwandan genocide saw kill orders broadcast by radio and it happened in South Sudan, 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 that former Vice President Riek Machar, who is now a rebel leader, comes from.

Lanzer said thousands of civilians from several ethnic groups are streaming to the U.N. peacekeeping base in Bentiu because many believe 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, the 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 mobile phones and a short while later gunmen began killing, both inside the mosque and inside the city hospital.

If you were not Nuer nothing could save you. The gunmen killed wantonly, including children and the elderly, Lanzer said.

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

British Ambassador Ian Hughes on Tuesday said 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.

U.N. officials began helping to clear the bodies from the streets and city buildings. Lanzer arrived in Bentiu on the third day of that operation but still counted 150 bodies. He said the U.N. is documenting the killings and will soon have “a pretty good grasp” on the precise number killed.

The violence is only one part of a dual crisis 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 when residents should be planting and cultivating.

The U.N. hasn’t spelled out clearly who exactly the victims were last week, but because ethnic Nuers carried out the killings it is likely that ethnic Dinkas were among the dead. The U.N. also said former residents of the Darfur region of Sudan were among those killed targeted. Nuer residents who refused to take part in the attacks were also killed.

The U.N. has been warning of mounting evidence of ethnically-targeted killings in the world’s newest nation as both government troops and rebel forces lose and gain territories in back-and-forth clashes. Despite a ceasefire signed earlier this year, both sides continue to trade allegations over human rights violations.

Though thousands of people are cramming into the U.N. base in search of shelter, they may not even be safe there. Last week an angry mob attacked a U.N. base in Bor and killed about 60 people. In that case, ethnic Nuers sheltering inside bore the brunt of the attack.

Gorgeu said such a potential attack is a major concern for the safety of his staff but that he cannot abandon the civilians in need.

“All this violence, if you look at Bor, if you look at Bentiu, it’s a major, major concern. We can see the level of violence is having an unacceptably high cost on the civilian population and this must be addressed,” Gorgeu said.