The U.N. Security Council voted unanimously Tuesday to nearly double the number of peace­keepers in the world’s newest nation to more than 14,000 and urged swift action to end a violent political and ethnic conflict that threatens to become a full-blown civil war.

Amid reports of mass graves, extrajudicial killings and rapes, tens of thousands of civilians have sought refuge in U.N. base camps that in some cases were described as under siege from ethnically motivated fighters. Hundreds of people, if not several thousand, have been killed, according to witnesses and U.N. officials.

There appeared to be no sign of a rapprochement between the central players in the crisis: President Salva Kiir, an ethnic Dinka, and former vice president Riek Machar, who is a Nuer, as the ethnic killings threaten to overwhelm U.N., U.S. and African efforts to end the violence.

In telephone calls Tuesday to Kiir and Machar, Secretary of State John F. Kerry urged them “to accept a cessation of hostilities and begin mediated talks,” State Department spokeswoman Jen Psaki said. The United States has been the primary international backer of South Sudan since its 2011 independence from Sudan.

The U.S. Africa Command moved about 50 Marines to neighboring Uganda, better positioning them to take action to protect U.S. facilities and personnel, if needed. The Marines were part of a 150-strong contingent deployed Monday from their base in Spain, along with transport and refueling aircraft, to Djibouti, on the Horn of Africa.

Meanwhile, South Sudanese government forces launched an offensive to retake the town of Bor, which was seized by Machar loyalists, potentially sparking more tit-for-tat attacks.

“There are definitely ethnic undertones to what is happening,” said Toby Lanzer, the deputy special representative to the U.N. mission in South Sudan. “But this is a political struggle within the ruling party. It’s actually by addressing that that we are going to be able to get things under control.”

A campaign of violence

The crisis was sparked by fighting between Dinka and Nuer soldiers. Kiir then accused Machar of trying to orchestrate a coup. Machar denies the charge but is leading a rebellion that has seized vital parts of South Sudan, including Bentiu, the capital of oil-rich Unity state.

The fighting has spread to five of the country’s 10 states, including Upper Nile state, another oil-producing region.

The Security Council action followed a report by U.N. High Commissioner for Human Rights Navi Pillay that at least one mass grave has been discovered in Bentiu, “and there are reportedly at least two other mass graves,” near Juba, the capital and South Sudan’s largest city.

“There is a palpable fear among civilians of both Dinka and Nuer backgrounds that they will be killed on the basis of their ethnicity,” Pillay said.

Nuer soldiers and gangs have reportedly targeted Dinka in Bor and in Bentiu, as well as some in Juba, killing many and forcing tens of thousands to flee their homes and seek protection with the United Nations.

In interviews over the past two days, more than two dozen displaced Nuer civilians, witnesses and victims described a campaign of targeted killings, rapes and beatings by Dinka soldiers. The violence has included the alleged killing of scores of young Nuer in a secret detention facility, their bodies buried in four shallow graves.

Witnesses said that Nuer men have been rounded up across Juba and that many were thrown in prisons for days, beaten with rifle butts or killed on the spot. Some had their hands tied up with wire, their arms and heads slashed with machetes, witnesses said. Dinka soldiers reportedly also set fire to and looted Nuer houses.

Three-year-old Nyajing Gadet was among the victims.

Last week, Dinka government soldiers arrived in her Juba neighborhood, a once-mixed area of Nuers and Dinkas called New Site, to hunt down Nuer house by house, her mother recalled. Dinka neighbors pointed the soldiers to the family’s home.

Soldiers fired through the walls and windows. A bullet grazed Nyajing’s head, spilling blood down her tiny face, as her father held her in his arms.

“They didn’t care if they killed a child,” said her mother, Elizabeth Nakiru, cradling Nyajing, who had a thick white bandage wrapped around her head. Both were inside a crowded tent in a U.N. compound where they had sought shelter, along with thousands of other Nuer. “They were firing on anyone who was Nuer.”

The South Sudanese military acknowledged Tuesday that abuses against Nuer civilians had taken place and has ordered a probe of the army — still referred to as the Sudan People’s Liberation Army, or SPLA — as well as police and security units involved in the operations last week.

“The SPLA we cannot say is perfect. There might be people there who are not properly oriented as national soldiers,” said Col. Philip Aguer, a spokesman for the military. “For us to build a nation, we have to carry out a serious investigation. This is bad behavior, and it will create a big hole in the body of the military.”

In a news conference, U.N. Secretary General Ban Ki-moon warned South Sudanese leaders that “targeted attacks against civilians and against U.N. personnel . . . could constitute war crimes or crimes against humanity.”

Lanzer told the BBC in an interview that the dead numbered in the “thousands.” One peacekeeper has been killed, and several have been wounded. On Saturday, four U.S. soldiers were wounded when an air mission to evacuate American citizens was fired upon in Bor.

Although 380 U.S. citizens have been evacuated, the State Department said it has no fixed count of how many remain — many of them are humanitarian workers — because they may not have registered with the U.S. Embassy here or may have left the country on their own.

The U.N. resolution authorizes Ban to temporarily transfer — with the permission of their governments — troops assigned to other peacekeeping and stabilization missions in Africa, including in Sudan, Liberia, Ivory Coast and Congo. He said he has reached out to the African Union and countries such as Ethi­o­pia and Rwanda, traditional suppliers of peacekeeping troops on the continent, and made appeals to Pakistan, Bangladesh and Nepal.

Ban said he has also been seeking resources to assist the new troops, whose mission is to protect civilians, including with “attack helicopters, utility helicopters and transport airplanes.” A Pentagon spokesman said he had no knowledge of a request for U.S. military assistance for the peacekeeping force.

Displaced and helpless

Deng Wang, 34, remembers being rounded up by Dinka soldiers and taken to a grass-thatched house in New Site. He was thrown in a room with scores of other Nuer men, perhaps as many as 200, he said. Each night, he said, soldiers would take a handful of them outside. Then, gunshots would follow. The men never returned.

By Sunday, he and eight others were the only survivors. That’s when a group of South Sudanese national security officials arrived and released them.

“When we walked outside, we saw dead bodies” behind the compound in open, shallow graves, Wang said.

Chiok Ring, 32, was stopped by Dinka soldiers in another part of the city. He and four other Nuer men, including his brother, were in his car, Ring recalled. They were easy to spot: all had six parallel horizontal lines etched across their forehead with a razor, part of the Nuer initiation into adulthood.

One soldier barked: “You are Nuer. Come out.”

“Then, they started to shoot at us,” Ring said. “My brother and my three friends were killed. I ran to a church and hid. There were women and children there, so the soldiers did not enter.”

Ring made his way to the main U.N. base in Juba, joining an estimated 20,000 people who live in a sea of crowded tents, many made from blankets and fixed to muddy ground. They sleep on dirty mattresses and dry the few clothes they possess on barbed wire. They depend on aid workers for food and water. The stench of mud and sewage wafts through the sanctuary, which aid workers say has become a breeding zone for malaria and other illnesses.

Most of the ethnic attacks have targeted Nuer men of fighting age, although in some cases soldiers appeared to fire randomly into houses occupied by Nuer and assault Nuer women and children. Upon arriving at the U.N. compound in Juba last week, victims told aid workers that sexual assaults had occurred.

“When I did the first assessment, they reported a lot of rapes outside,” said Christine Bimansha, a physician with Doctors Without Borders, which is caring for the displaced.

Thony Wan, 27, was rounded up with eight Nuer friends, all students like him, in the neighborhood of Mangateen. The Dinka soldiers demanded to know whether Wan and the others were soldiers. One placed his gun to the head of one of Wan’s friends and ordered him to “confess” that he was a soldier, Wan recalled. But a Dinka neighbor vouched for them; the soldiers grabbed the youths’ phones and money and left to find other Nuer.

Minutes later, “we heard gunshots,” Wan recalled. A woman running from the direction of the gunfire informed them that the soldiers had killed five Nuer men. They fled the neighborhood, arriving at the U.N. base with “only the clothes we wore,” Wan said.

Many of the displaced have relatives missing. Khat Thoch, 29, said his step-brother was picked up by soldiers last week. Since then, the step-brother’s cellphone has been switched off. He was an American citizen who lived in Nebraska before returning to live in South Sudan in 2008, Thoch said. “Maybe, he has been killed,” Thoch added.

Inside a white tent, filled with about 40 people, David Gan, 35, sat next to two relatives. All three relatives had bullet wounds. Dinka soldiers, they said, fired into their house, killing five housemates. They were the only survivors.

“If I have access to a gun, I will kill a Dinka,” said Gan, whose left arm was in a sling.

There are few Nuers left in New Site. “Some have gone to the U.N. base, others have simply run away,” said Robert Makwachu, a Dinka soldier, guarding the area with pride.

Moments later, a Nuer soldier drove by on a motorcycle, and the two men exchanged angry words. After the Nuer soldier left, Makwachu shook his head. “He may have a gun, but he is powerless,” Makwachu said.

Nyajing’s mother tried to return home last week. She had left some flour in their house, and she wanted to retrieve it to feed her children. But when she arrived, she found a padlock on the door. Her Dinka neighbors had seized her house, she said.

When she tried to break in, two Dinka soldiers spotted her and assaulted her, as her Dinka neighbors watched without intervening. “They dragged me and beat me,” said Nakiru, a wound on her right cheekbone still raw. But she added, “They did not tamper with my body.”

Like many of her fellow Nuer, she fears leaving the U.N. compound.

“Going back home will be like committing suicide,” she said.

DeYoung reported from Washington.