US President Barack Obama has warned that South Sudan is on the "precipice" of a civil war, after clashes in the capital Juba spread around the country.
He said 45 military personnel had been deployed to South Sudan on Wednesday to protect American citizens and property.
At least 500 people are believed to have died since last weekend, when President Salva Kiir accused his ex-deputy Riek Machar of a failed coup.
An estimated 34,000 people have taken refuge at United Nations compounds.
Two Indian peacekeepers were killed on Thursday when a UN base sheltering refugees at Akobo, Jonglei state, came under attack, the UN mission to the country, Unmiss, said on Friday. An injured Indian soldier was taken to hospital, it said.
Earlier reports said three Indian peacekeepers had been killed in the attack.
UN deputy spokesman Farhan Haq said more casualties were feared, and he did not know the fate of more than 30 Dinka civilians sheltering at the base.
Security at the compound has since been increased.
Sudan suffered a 22-year civil war that left more than a million people dead before the South became independent in 2011.
The recent unrest has pitted gangs from the Nuer ethnic group of Mr Machar against Dinkas - the majority group to which Mr Kiir belongs.
"South Sudan stands at the precipice. Recent fighting threatens to plunge South Sudan back into the dark days of its past," President Obama said in a letter to Congress.
"Inflammatory rhetoric and targeted violence must cease. All sides must listen to the wise counsel of their neighbours, commit to dialogue and take immediate steps to urge calm and support reconciliation."
President Kiir has blamed the violence on soldiers who support Mr Machar. Mr Machar, who was sacked by Mr Kiir in July, has denied trying to stage a coup. Mr Machar's whereabouts are unknown.
Despite calls for calm, the rival groups have been fighting fierce gun-battles.
African Union spokesman Ateny Wek Ateny said on Thursday that the town of Bor, the Jonglei state capital, north of Juba, was in the hands of forces loyal to Mr Machar.
"They control the town but government forces are trying to retake it," he said.
A health official in Juba told the BBC that six doctors in Bor had fled the town after the hospital came under fire from shelling. One of them texted him to say they were still hiding in the bush.
Heavy fighting between Nuer and Dinka soldiers has also been reported near Bentiu, capital of oil-producing Unity State.
A UN spokesman told Reuters news agency more than 500 civilians - and 27 soldiers loyal to President Kiir - had sought refuge at a UN compound there.
A number of countries were preparing to evacuate their nationals. Britain was sending another flight on Friday, a day after a military transport plane evacuated 182 people, including 53 Britons, to Uganda.
Uganda has sent a small contingent of troops to help pull out its nationals.
A US plane was also due to take Americans out of the country. And China's National Petroleum Company was evacuating oil workers to Juba.
Jonglei state has witnessed some of the worst violence since South Sudan became independent, with hundreds killed in clashes between rival heavily-armed ethnic militias sparked by cattle-rustling.
Following decades of conflict, weapons such as machineguns are widely available in much of South Sudan.
In a statement, the UN mission in the country, Unmiss, said conditions for displaced people in Juba and Bor were "challenging".
It said in some of the UN bases, some people had been able "to construct basic shelters with available materials, but many have no or limited access to shelter".
A delegation of East African foreign ministers has travelled to Juba to try to mediate in the crisis.
South Sudan's government insists the clashes are over power and politics, not between ethnic groups.
The oil-rich country has struggled to achieve a stable government since becoming independent.