Residents of Juba arrive at the UN compound on December 20, 2013, where they sought shelter.
Attackers slaughtered at least 20 civilians sheltering in a UN base in South Sudan in an attack in which three Indian peacekeepers died, the UN said on Friday.
Some 2,000 armed ethnic Nuer youths stormed the UN base at Akobo in Jonglei state where 36 ethnic Dinka civilians had sought refuge, a UN statement said.
The UN Mission in South Sudan (UNMISS) said in a statement that "at least 20 of the civilians who sought refuge inside the base were killed during the attack".
The three Indian peacekeepers were killed "defending the base against the assailants".
"An estimated 2,000 armed youths believed to be of Lou Nuer ethnicity surrounded the UNMISS base in Akobo and opened fire in the direction of South Sudanese civilians of Dinka ethnic origin who had sought refuge inside," said UNMISS.
"While trying to open negotiations with the assailants, UNMISS peacekeepers stationed inside the compound came under sustained attack."
The attackers seized all of the weapons, ammunition and other supplies in the base that was eventually retaken by the South Sudan People's Liberation Army (SPLA).
The UN envoy to South Sudan, Hilde Johnson, slammed the attack as "a criminal act for which the responsible must be held accountable".
There were 43 Indian peacekeepers at the small UN base at the time of the attack. The 40 who were unhurt were taken to a nearby SPLA camp.
Edmond Mulet, assistant secretary general for UN peacekeeping, told emergency Security Council talks of mounting ethnic clashes among Dinka followers of President Salva Kiir and Nuer supporters of former vice president Riek Machar.
Mulet said there had been at least 500 deaths in the capital Juba since the fighting started on Sunday and the United Nations was still verifying the toll in the rest of the country, according to diplomats at the closed meeting.
He said there were more than 35,000 people in UN bases around the country, including 20,000 at two compounds in Juba and 14,000 at another in Pibor in Jonglei state.
The UN mission is also sheltering civilians in Bentiu, the main town in the oil producing Unity state.
Mulet told of one attack on a Bentiu oil facility by Nuer youth.
Witnesses said the youths ordered all employees to come out of the facility, told all ethnic Dinka to step forward and then killed them, Mulet was quoted as telling the Security Council meeting.
At least five plant employees were killed, according to UN sources in South Sudan.
A group of African foreign ministers is in Juba and met with Salva Kiir on Friday. Mulet said that the president had agreed to "unconditional dialogue" with his opponent to end the crisis.
Salva Kiir has accused Riek Machar of staging an attempted coup. The former vice president has denied the charge, but his whereabouts are unknown.