The UN Security Council on Tuesday nearly doubled the size of its peacekeeping force in South Sudan as mass killings on ethnic grounds were reported between warring factions in the East African country.

The 15-member Security Council unanimously adopted a resolution for 5,500 peacekeepers to bolster the UN Mission in South Sudan (UNMISS), which already has about 7,000 military members. An additional 1,300 civilian police were to join the mission.

The peacekeepers are authorized to use force to carry out their mandate to protect civilians and facilitate delivery of humanitarian aid to the tens of thousands of civilians needing help, due to displacement caused by fighting that erupted December 15.

UN Secretary General Ban Ki-moon called for an end to hostilities and for dialogue between the two sides. He said the United Nations was working with regional leaders and parties on the ground to establish the basis for negotiations.

“There is no military solution to this conflict,” Mr. Ban said. “This is a political crisis, which requires a peaceful political solution.”

Navi Pillay, UN high commissioner for human rights, said her office has documented in recent days “mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions.” She said a mass grave was discovered in Bentiu in Unity State, and there were at least two other mass graves in Juba, the capital.

Ms. Pillay called on South Sudan’s leadership to denounce violence as civilians were being caught in the crossfire.

Citing the two largest ethnic groups at the heart of the conflict, she said there was “a palpable fear among civilians of both Dinka and Nuer backgrounds that they will be killed on the basis of their ethnicity.” With rival factions within the army fighting, hundreds of people have been killed as South Sudan teeters on the brink of civil war.

A UN official said on Tuesday that 81,000 people have been displaced since the fighting started, including some 45,000 seeking protection at UN bases.

South Sudanese President Salve Kiir has accused his rival Riek Machar of attempting to stage a coup. Mr. Machar says Mr. Kiir has become dictatorial and has made clear his wish to oust the incumbent and become president himself.

Mr. Kiir is a Dinka, South Sudan’s largest ethnic group, while Mr. Machar is a Nuer.

Jonglei, Unity and parts of Upper Nile states have fallen to Mr. Machar loyalist army units. Unity and Upper Nile are oil-producing regions along the border with Sudan.

African countries have begun to push for negotiations between the Mr. Kiir and Mr. Machar camps, but the terms are unclear and there is no timeline for the talks.

Aid workers from Doctors Without Borders (MSF) said more refugees were flooding into Juba.

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