Battles between rival ethnic groups spread across South Sudan on Friday as foreign governments scrambled to get their nationals away from reported slaughters and a refugee buildup.
African ministers pushed President Salva Kiir to start talks with his former vice president Riek Machar.
But the death toll mounted and the United Nations strongly condemned an attack on one of its bases in which at least 11 civilians and two Indian peacekeepers were killed.
UN officials reported that up to 3,000 armed youths had gathered around another camp at Bor in Jonglei state where 14,000 people have sought refuge.
Six days into the battles between followers of Kiir, an ethnic Dinka, and Machar, a Nuer, at least 500 people have been killed in the capital Juba alone.
The UN says more than 35,000 people are sheltering in its compounds across the country and Juba airport was packed with foreigners scrambling to escape the chaos.
Britain sent a second military transporter to Juba on Friday to evacuate 93 people from the country.
China National Petroleum Corporation started pulling its workers out of South Sudan's oil fields and other Chinese firms followed the move, China's foreign ministry said.
The United States has deployed 45 troops in Juba to protect US property and closed down its embassy. US President Barack Obama has warned that South Sudan "stands at the precipice."
Uganda said it also deployed special forces to get its nationals out of Juba and help secure the city.
UN Secretary General Ban Ki-moon for the second time in as many days, denounced the violence, and issued an appeal Friday for renewed efforts to restore peace in South Sudan.
"The Secretary General reiterates his call for all parties to exercise restraint, and to cease hostilities," the UN leader said, one day after the deadly attack on one of its bases.
Violence erupted after a meeting last week of the National Liberation Council of the failed to lessen tensions in the ruling party.
Ban urged leaders of the Sudanese People's Liberation Movement "to resolve their personal differences through dialogue immediately."
The ethnic divide grew with reports from around the country of killings of Dinka and Nuer.
About 2,000 armed youths, believed to be Nuer, stormed a UN base in Akobo on Thursday, killing "at least 11" Dinka civilians and two Indian peacekeepers, the UN Mission in South Sudan said.
A third Indian peacekeeper was badly wounded and taken to Juba for treatment, an UNMISS statement said.
About 36 ethnic Dinka civilians had sought refuge in the base where 43 Indian peacekeepers and six police advisors were stationed.
The two Indian soldiers were killed "defending the base against the assailants" who launched "a sustained attack," UNMISS said.
The armed youths fired on the civilians who had taken refuge and seized all of the weapons and ammunition in the base before fleeing.
Meanwhile, between 2,000 and 3,000 armed youths were reported close to a UN base at Bor, the main town in Jonglei state, where 14,000 people have fled, France's UN envoy Gerard Araud said after emergency UN Security Council talks on the crisis.
Araud, Security Council president for December, said there was "heavy fighting" in Bor and worries about where the youths might be heading.
Troops loyal to Machar seized Bor on Wednesday.
Edmond Mulet, assistant UN secretary general for peacekeeping, briefed the closed Security Council meeting on still more clashes, diplomats said.
Nuer youth entered one oil facility at Bentiu in Unity state and ordered all ethnic Dinka employees to step forward and then killed them, Mulet quoted witnesses as saying.
At least five 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 the president had agreed to "unconditional dialogue" with his opponent to end the crisis.
Kiir has accused Machar of staging an attempted coup. The former vice president has denied the charge, but his whereabouts is unknown.
Mulet said there had been at least 500 deaths in Juba since the fighting started on Sunday and the United Nations was still verifying the toll in the rest of the country.
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 Unity state.
South Sudan became independent from Sudan in 2011 as part of a process after a two decade civil war with the Khartoum government that left two million dead.
It has never been able to heal its own ethnic rivalry, however.
Obama has warned that hopes for South Sudan that accompanied its independence are now "at risk."