A mass grave containing some 75 bodies has been found in South Sudan's Unity State and two other mass graves have been reported in the capital Juba after ethnic violence, the United Nations said on Tuesday.
U.N. High Commissioner for Human Rights
Navi Pillay called on both sides to protect civilians and warned that political and military leaders could be held to account for crimes.
"Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days," Pillay said in a statement
. "We have discovered a mass grave in Bentiu, in Unity State, and there are reportedly at least two other mass graves in Juba."
President Salva Kiir and rebel leader Riek Machar, his former deputy, have both indicated they were ready to talk to try to end a deepening conflict that has killed hundreds of people since it erupted this month.
Members of the South Sudan rebel delegation attend the opening ceremony of South Sudan's peace negotiations, Addis Ababa, Ethiopia, January 4, 2014.
Taban Deng Gai, left, head of the rebel delegation and South Sudan's leader of the government delegation, Nhial Deng Nhial, attend the opening ceremony of South Sudan's peace negotiations, Addis Ababa, Ethiopia, January 4, 2014.
Unidentified members of the delegation from the South Sudan government and western observers meet at the Sheraton Hotel, Addis Ababa, Ethiopia, January 4, 2014.
A displaced mother and her baby, one of the few to have a mosquito net, wake up at a refugee camp, Awerial, South Sudan, Jan. 2, 2014.
A young displaced girl carries a bucket of water back to her makeshift shelter at a United Nations compound. The compound has become home to thousands of people displaced by the recent fighting, Juba, South Sudan, Dec. 31, 2013.
Displaced people gather inside a mosquito net tent as they flee from the fighting between the South Sudanese army and rebels in Bor town, in Juba, South Sudan, Dec. 30, 2013.
A displaced woman hangs up laundry on the plastic sheeting wall of a latrine at a United Nations compound, Juba, South Sudan, Dec. 31, 2013.
Yared, 2, is held by his mother, Madhn, who fled from the town of Bor a few days ago. She receives medicine for her child at a Medecins Sans Frontieres (MSF) medical tent, at a United Nations compound, Juba, South Sudan, Dec. 31, 2013.
A young displaced boy rests on the wheel arch of a water truck while others fill containers from it, at a United Nations compound, Juba, South Africa, Dec. 31, 2013.
A family makes tea outside their makeshift shelter at a United Nations compound, Juba, South Sudan, Dec. 31, 2013.
A general view of a camp for displaced people set up in a United Nations compound in Bor, South Sudan, Dec. 25, 2013.
South Sudan army soldiers hold their weapons as they ride on a truck in Bor, Dec. 25, 2013.
Western powers and east African states, anxious to prevent the fighting from destabilizing a particularly fragile region, have tried to mediate between Machar, who hails from the Nuer tribe, and Kiir, a Dinka.
Spokeswoman Ravina Shamdasani said that the bodies of 75 soldiers of the Sudan People's Liberation Army were believed to be in the mass grave in Bentiu visited by U.N. rights officers.
"They are reportedly all of Dinka ethnicity," Shamdasani told Reuters in Geneva, adding that the U.N. team had been unable to verify the numbers or identities.
U.N. rights officers had not yet been able to visit the sites of two other mass graves, Jebel-Kujur and Newside, near Eden, both in Juba, she said.
Pillay, a former U.N. war crimes judge, voiced deep concern about the safety of those arrested who are being held in unknown locations, including "several hundred civilians who were reportedly arrested during house-to-house searches and from various hotels in Juba".
Hundreds of members of the South Sudan National Police Service were allegedly ordered to be disarmed and arrested from police stations across Juba, she said.
U.N. Secretary-General Ban Ki-moon sought urgently on Monday to nearly double the size of the U.N. peacekeeping force in the country.