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.
On Thursday three Indian peacekeepers died in an attack on a UN compound.
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.
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 to which Mr Machar belongs 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."
India's UN ambassador, Ashok Mukerji, earlier announced the peacekeepers' deaths at a UN meeting in New York.
He said they were "targeted and killed" during Thursday's attack by ethnic Nuer youths on the base at Akobo, Jonglei state.
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.
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.
Despite calls for calm, the rival groups have been fighting fierce gun-battles over the town of Bor, in Jonglei state north of Juba.
African Union spokesman Ateny Wek Ateny said on Thursday that Bor was in the hands of forces loyal to Mr Machar.
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"They control the town but government forces are trying to retake it," he said.
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.
BBC
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