NAIROBI, Kenya - The townsfolk believed the mosque was safe. They crammed inside as rebel forces in South Sudan took control of the town from government troops. But it wasn't safe. Robbers grabbed their cash and mobile phones. Then gunmen came and opened fire on everyone, young and old.

The U.N. says hundreds of civilians were killed in the massacre last week in Bentiu, the capital of South Sudan's oil-producing Unity state, a tragic reflection of longstanding ethnic hostilities in the world's newest country.

"Piles and piles" of bodies were left behind after the shootings, said Toby Lanzer, the top U.N. aid official in South Sudan. Many were in the mosque, others were in the hospital. Still more littered the streets. The violence appears to have been incited in part by calls on the radio for revenge attacks, including rapes.

The attack, which targeted members of certain ethnic groups, was a disturbing echo of what happened two decades ago in another country in eastern Africa. Rwanda is marking the 20th anniversary this month of a genocide that killed an estimated 1 million people and also saw orders to kill broadcast over the radio.

Thousands of people have been killed in violence in South Sudan since December, when presidential guards splintered and fought along ethnic lines. The violence later spread across the country as soldiers loyal to President Salva Kiir, an ethnic Dinka, tried to put down a rebellion led by Riek Machar, the former vice-president and an ethnic Nuer.

But Lanzer told The Associated Press in a phone interview Tuesday that the April 15-16 mass killings, carried out by Nuers, are "quite possibly a game-changer" in the conflict.

"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."

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 22,000 people - up from 4,500 at the start of April - but can supply only one litre 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.

Raphael Gorgeu, the head of Doctors Without Borders in South Sudan, said people will die inside the U.N. base in the 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.

The U.N. hasn't spelled out clearly who exactly the victims were, but it is likely that ethnic Dinkas were among the dead. If you were not Nuer, then nothing could save you. And even Nuers who refused to take part in the attacks were killed, according to the U.N., as were former residents of the Darfur region of Sudan.

The gunmen killed wantonly, including children and the elderly, Lanzer said.

U.N. officials began helping to clear the bodies from the streets and city buildings after the bloodshed. 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.

At U.N. headquarters in New York, spokesman Stephane Dujarric said Tuesday that many bodies remain by the side of the main road between Bentiu and Rubkona, another town in Unity state, and that the Rubkona market continues to be looted.

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 said Tuesday that the killings are a clear violation of international law. He said those behind the atrocities and those inciting them will be held to account.

The violence is only one part of a dual crisis in South Sudan, a landlocked country that gained its independence from Sudan in 2011. Because of the fighting, more than 1 million people have fled their homes, and few residents are tending crops. Lanzer cited a severe risk of famine in the months ahead.

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The U.N. has been warning of mounting evidence of ethnically targeted killings as both government troops and rebel forces lose and gain territories in back-and-forth clashes. A cease-fire signed earlier this year has done little to quell violence.

Though thousands of people are cramming into the U.N. base in Bentiu, they may not even be safe there. Dujarric said four rockets were fired at the base Thursday, including two that exploded within the compound and one just outside, wounding two people who had sought refuge,.

Also last week, an angry mob attacked a U.N. base in Bor, a town in Jonglei state, killing about 60 people. In that case, ethnic Nuers sheltering inside bore the brunt of the attack. Dujarric said the U.N. mission in South Sudan reports that the situation in Bor remains "tense."

Asked how the United Nations could protect the 22,000 people at the base in Bentiu, given what happened in Bor, Dujarric said there are 500 U.N. peacekeepers in Bentiu. He reminded South Sudan's government that it has a responsibility to protect civilians and that all armed groups have a responsibility to avoid civilian casualties.