Fierce clashes erupted in South Sudan on Wednesday as rebels sought to seize control of oil-rich areas, two days after U.N. officials accused them of killing hundreds of civilians in ethnically motivated attacks.

Fighting raged in three states despite a cease-fire agreement reached in January, said Col. Philip Aguer, South Sudan’s military spokesman, raising the likelihood of more tit-for-tat attacks and civilian casualties. Already, thousands have been killed. More than a million people have fled their homes since the conflict broke out in early December, tearing apart the world’s newest nation.

The targeting of civilians underscores the extent to which the fighting has become an ethnic conflict between South Sudan’s two largest tribes, the Dinka and the Nuer.

The violence has also undermined more than a decade of diplomacy and assistance by the United States and its Western allies to end Sudan’s 22-year civil war and forge an independent South Sudan three years ago.

The tensions pit President Salva Kiir, an ethnic Dinka, against his former vice president, Riek Machar, an ethnic Nuer. The crisis began last year when Kiir accused Machar of trying to orchestrate a coup. Machar denied the charges but then launched a rebellion.

Aguer said the rebels had received new weapons and ammunition from Sudan and had recruited Arab militias backed by Sudan’s government to help fight.

“The new weapons and the new recruits, mainly from Sudan — this is why the rebels are fighting in the border areas, in Upper Nile and Unity” states and in Jonglei, Aguer said in a telephone interview. “They have rejected the cessation-of-hostilities pact.”

The United Nations has accused Machar’s forces of massacring hundreds of civilians on April 15 and 16 after recapturing the oil-producing town of Bentiu. Fighters sought out people based on their ethnicity and nationality and slaughtered them, according to a statement from the U.N. mission in South Sudan. The dead included many who had sought refuge at a mosque, a Catholic church and a hospital.

Bodies littered the streets in Bentiu, according to video taken by the Al Jazeera network Tuesday. More bodies were stacked inside the mosque. Bulldozers buried the dead in mass graves. The number seeking refuge at the U.N. base in Bentiu has soared from 5,000 to 22,000 over the past two weeks, according to U.N. officials.

A senior rebel official denied that Machar’s forces had massacred civilians and said those killed were members of armed groups working with the government.

“It was too soon for the United Nations to judge that the rebels had killed civilians,” said Gideon Gatpan Thoar, a senior rebel official based in Nairobi. “There were no civilians. . . . These people had guns.”

He also denied that the rebels had received new weapons and militia recruits from Sudan.

‘An abomination’

The Obama administration called the attacks “an abomination” and said they “shock the conscience.”

“They are a betrayal of the trust the South Sudanese people have put in their leaders,” the White House said in a statement. “This is exactly the violence and suffering the South Sudanese people fought for decades to escape.”

On Wednesday, Human Rights Watch joined the outpouring of condemnation by calling for a U.N. fact-finding mission to investigate the atrocities and impose sanctions on those responsible — not only for the attacks in Bentiu, but also for an assault on a U.N. base in Bor last week that killed dozens of people.

“The killing of more than 50 people in a UN base in Bor and the gruesome massacres of hundreds of civilians in Bentiu shows that ethnically motivated brutality against civilians is spiraling out of control,” said Daniel Bekele, Africa director at Human Rights Watch. “The U.N. Security Council needs to act decisively to impress on the warring parties targeting civilians in South Sudan that they will pay the price for their crimes.”

Also alarming, the United Nations said, was the use of Radio Bentiu FM to broadcast hate speech, evoking memories of Rwanda’s 1994 genocide, in which ethnic Hutu extremists used radio and television stations to order the killings of ethnic Tutsis. Via Radio Bentiu, the rebels urged rival ethnic groups to leave town and called on their supporters to “commit vengeful sexual violence against women” from “another community,” presumably referring to ethnic Dinkas.

Fractured alliance

Kiir and Machar have long been rivals inside the Sudan People’s Liberation Movement, but they worked together to achieve South Sudan’s independence from Sudan three years ago. Since then, though, their relationship in the coalition government has been strained and their mutual suspicions have deepened. Last summer, Kiir fired Machar from his position.

What began as a political tussle for power swiftly took on ethnic dimensions. In early December, scores of ethnic Nuer civilians were killed, executed, raped and beaten by ethnic Dinka soldiers loyal to Kiir. Since then, the two sides have fought for territory, mostly in oil-producing areas such as Bentiu, even as they continue to engage in peace talks.

South Sudan receives 98 percent of its revenue from exporting crude oil, and both sides view oil as vital for their political and economic survival.

Many of the 5,000 people who had sought refuge at the U.N. base in Bor before last week’s attack by mobs of youths were Nuer.

It was unclear whether the attacks in Bentiu were in retaliation for the deaths in Bor because the victims were from many ethnic groups. The United Nations also said that “several Nuer men, women and children were killed for hiding and declining to join other Nuers who had gone out to cheer” the rebels as they entered Bentiu.