UNITED NATIONS — A new U.N. report says no one has been held accountable in a massacre in South Sudan last year that raised international alarm and memories of the Rwandan genocide.

The report released Friday said more than 350 civilians were killed in two bursts of violence along ethnic lines as fighting between government troops and rebels reached a bloody height last April. The attacks in Bentiu and Bor led the U.N. Security Council to express "horror" and threaten sanctions, while the top U.N. aid official there reported "piles and piles" of bodies in the streets.

The new report says the targeted attacks on civilians in Bentiu by rebels, and in Bor by government supporters, could amount to war crimes.

Oil-rich South Sudan remains in turmoil as three cease-fire agreements have failed.

The world's newest country, which split from Sudan in 2011, fell into civil war just over a year ago after President Salva Kiir, an ethnic Dinka, accused former Vice President Reik Machar, an ethnic Nuer, of trying to oust him in a coup.

The U.N. says tens of thousands of people have died and 1.9 million more have been displaced by the fighting. More than 100,000 civilians have sheltered for months in U.N. bases around the South Sudan. The Security Council has blamed the country's "political, security and humanitarian catastrophe" on its feuding leaders.

The Bentiu massacre centered on a mosque where civilians crammed inside while trying to flee rebel fighters. Others were killed in a hospital where they tried to hide, or on the streets. The new report says more than 280 people were killed in the mosque alone.

Two days later, a mob attacked a U.N. base in Bor where thousands of displaced people were sheltering. The report says at least 47 civilians were killed. Though most of the attackers wore civilian clothing, "there is ample information suggesting that government security forces were present before and during the attack and failed to stop the violence," the report says.

The report calls for an end to fighting and to human rights abuses in South Sudan, and it urges the international community to support peace talks and the protection of civilians.

South Sudan's government last week announced that it will hold general elections between May and July and give amnesty to the rebels to encourage their participation, but the rebels quickly rejected the plan and said they would not take part.