Two United Nations peacekeepers from India have been killed and one wounded in an attack on a UN base in South Sudan, a UN spokesman said.
The UN said attackers from the country's second-largest ethnic group forced their way into the Akobo base in conflict-wracked Jonglei state on Thursday, pursuing civilians from a rival ethnic group who had taken refuge there.
India's UN envoy Asoke Mukerji said earlier that three peacekeepers were killed by armed ethnic Nuer youths, but UN mission later said the third peacekeeper was only injured and evacuated to a UN medical facility in Malakal.
Contact with the base was lost after the assault and UN deputy spokesman Farhan Haq said the fate of more than 30 ethnic Dinka civilians sheltering at the base was not known.
A statement said UN Secretary-General Ban Ki-moon was "appalled" to learn of the attack.
"There are indications that civilians may have been killed and wounded in the attack, but this remains to be verified. Should these reports prove true, those responsible must be held accountable for their crimes," the statement said.
Rapidly escalating ethnic violence has raised fears of instability in the world's newest country.
At 'precipice' of civil war
Meanwhile, Barack Obama, the United States president, called for an immediate end to the fighting in South Sudan on Thursday, warning the country stands at the "precipice" of civil war.
Recent fighting threatens to plunge South Sudan back into the dark days of its past.
Obama, who earlier announced he had deployed 45 troops to the violence-wracked country on Wednesday to protect US personnel and interests, warned that "recent fighting threatens to plunge South Sudan back into the dark days of its past".
"Fighting to settle political scores or to destabilise the government must stop immediately. Inflammatory rhetoric and targeted violence must cease," the US president added in his statement.
"All sides must listen to the wise counsel of their neighbours, commit to dialogue and take immediate steps to urge calm and support reconciliation. South Sudan's leaders must recognise that compromise with one's political enemy is difficult; but recovering from unchecked violence and unleashed hatred will prove much harder."
The attack on Indian peacekeepers on the UN base came after troops loyal to fugitive former vice president Riek Machar seized the town of Bor late on Wednesday, as fighting continued in eastern Jonglei state.
President Salva Kiir has blamed the bloodshed on a coup bid by his perennial rival Machar, who says the alleged overthrow was a fabrication to cover up a regime purge.
Machar denies coup
Talking to Al Jazeera's Hannah McNeish, Machar denied that any plot was carried out to coup Kiir.
"My life was in danger; my colleagues were being arrested for no reason. They are not plotters, it was not a coup. Nobody wants that," Machar said, claiming he was "used as a scapegoat" by Kiir to purge the ruling SPLM party of rivals to avoid reforming it.
Kiir coming from the majority Dinka people and Machar from the Nuer.
On Friday, Ethiopian Foreign Minister Tedros Adhanom said talks between Kiir and African mediators trying to broker a peace deal after six days of clashes between rival army factions were processing well.
"We had a very productive meeting with his excellency President Slava Kiir and we will continue consultations," Adhanom, leader of the Inter Governmental Authority on Development (IGAD) delegation, told reporters in Juba.
About 450 people have been killed in the capital, Juba, since battles broke out on Sunday, including around 100 soldiers, army spokesman Philip Aguer said.
The UN's humanitarian arm said on Friday that about 20,000 people were seeking shelter in two UN bases in the capital Juba, while at least 14,000 were seeking shelter in Bor, the capital of Jonglei state. Several hundred people are also trying to find shelter in Bentiu, the capital of Unity State, where South Sudan's oil fields are concentrated.