The UN's top human rights official arrived in South Sudan on Monday, officials said, amid mounting global outrage over the country's civil war and a wave of atrocities.
The UN's High Commissioner for Human Rights Navi Pillay and special envoy for the prevention of genocide Adama Dieng arrived in the capital Juba, a UN spokesman said.
The two are expected to remain in the country until Wednesday and hold meetings with South Sudan President Salva Kiir and other senior officials. They are due to tour massacre sites "if security conditions allow", UN Mission in South Sudan spokesman Joe Contreras said.
Last week the UN Security Council raised the threat of sanctions against both sides in the conflict -- the government forces loyal to President Kiir, and rebels backing former vice president Riek Machar.
Both sides have been implicated in atrocities and war crimes including massacres, rapes, attacks on UN bases sheltering civilians from ethnic violence and the recruitment of child soldiers.
In neighbouring Ethiopia, peace talks between the government and rebels were scheduled to resume on Monday, according to regional mediators, although previous rounds have achieved little and hopes of progress remain limited. A ceasefire signed in January quickly collapsed.
"Talks are set to start today," a diplomat from the IGAD regional bloc told AFP.
The head of the government peace delegation, Information Minister Michael Makuei, also said a resumption may be possible later in the day.
Earlier this month, the rebels were blamed for the killings of hundreds of people in the oil hub of Bentiu, and a pro-government mob killed dozens of civilians in an attack on a UN base in Bor.
More details on the Bentiu atrocities emerged on Monday, with the international aid agency Doctors Without Borders (known by its French acronym MSF) releasing fresh eyewitness testimony of the carnage.
"What I saw in Bentiu -- bodies of civilians strewn through the streets in grisly states of damage and decay, being eaten by dogs and birds -- was an affront to humanity," said Raphael Gorgeu, MSF's head of mission in the country.
"The violence in South Sudan has taken a particularly ugly turn, stripping people of their most basic human dignity. It is a terrible thing to witness."
Christopher Lockyear, MSF's operations manager for South Sudan, said staff had "heard tales of horrific brutality taking place on the hospital grounds" in Bentiu, and had credible information of up to 33 people killed in Bentiu State Hospital when the town fell to rebels.
"People who fled to the hospital in search of safety were selectively targeted based on their identities and loyalties. Once again in South Sudan, we see hospitals -- places that should be protected safe havens -- are increasingly places of attack and cruelty," he said.
Kiir's government has been fighting Machar's forces -- a mix of army defectors and ethnic militia -- since December 15.
The four-month-old war has left thousands and possibly tens of thousands of people dead, and forced over one million to flee their homes.
More than 78,000 civilians are currently crammed into eight UN bases in the country, while thousands have fled to neighbouring states, mainly Uganda and Ethiopia.
The violence has taken on an ethnic dimension, pitting Kiir's Dinka tribe against militia forces from Machar's Nuer people.