Fighting in South Sudan has killed up to 500 people, UN diplomats said Tuesday, and the United Nations fears the violence is "largely along ethnic lines."
The United States ordered its citizens to leave South Sudan immediately.
The president of the world's newest country has blamed the violence on a coup attempt by soldiers loyal to his former deputy, who belongs to a different ethnic group.
As many as 20,000 people have taken refuge with the UN mission in the capital, Juba, the president of the Security Council, French Ambassador Gerard Araud, told reporters.
Araud said the council received only "patchy information" in a special briefing Tuesday evening by the UN peacekeeping chief, with the cause of the violence yet unknown.
"We are extremely concerned," Araud said. "There is a heavy toll, that's obvious," he added.
UN diplomats said they were told that local sources in Juba put the death toll at 400 to 500, but that figure could not be verified.
Araud said that for the moment, civilians are not being targeted, and he described the violence as "infighting between soldiers."
Sporadic but heavy gunfire has been heard across Juba since Sunday as factions of the armed forces repeatedly clashed.
The office of UN chief Ban Ki-moon issued a statement saying the secretary-general is "deeply concerned" about "the risk of targeted violence against certain communities." He urged everyone to immediately cease their hostilities.
The US Embassy said in an advisory that Americans who choose to stay in South Sudan "should review their personal security situation and seriously reconsider their plans." The embassy suspended its normal operations.
President Salva Kiir told the nation on Monday that a group of soldiers loyal to former Vice President Riek Machar, whom he fired in July amid a power struggle, tried to take power by force but were defeated. Kiir then ordered a dusk-to-dawn curfew in the capital.
The alleged coup attempt took place Sunday when some soldiers raided the main army barracks' weapons store in Juba but were repelled by loyalists, sparking gunfights across the city, Foreign Minister Barnaba Marial Benjamin told The Associated Press.
Some officials had previously estimated the casualties could be in the hundreds.
There are "disturbing reports of ethnically-targeted killings," with most of the fighting pitting soldiers from Kiir's majority Dinka tribe against those from Machar's Nuer tribe, said Casie Copeland, the South Sudan analyst for the International Crisis Group.
"The fighting has been fierce and parts of Juba have been reduced to rubble," she said.
The South Sudanese military has arrested five political leaders with suspected links to the coup attempt, and many more are still being sought, Benjamin said. The government later said in a statement that 10 people have been arrested in connected with the alleged coup plot, and five remain at large, including Machar.
The US Embassy in Juba and the UN Mission in South Sudan have denied they are harboring Machar, Benjamin added.
The hunt for Machar, an influential politician who is one of the heroes of a brutal war for independence waged against Sudan, threatens to send the country into further political upheaval.
Machar, the deputy leader of the ruling party, said he would contest the presidency in 2015. He has openly criticised Kiir, saying if South Sudan is to be united it cannot tolerate "one man's rule or it cannot tolerate dictatorship."
The international community has repeatedly urged South Sudan's leaders to exercise restraint amid fears the violence could spark wider ethnic violence.
The UN chief told Kiir in a telephone call Tuesday that he expected him "to exercise real leadership at this critical moment, and to instill discipline in the ranks of the (Sudanese military) to stop this fighting among them," according to UN spokesman Martin Nesirky.
The oil-rich East African nation has been plagued by ethnic tension since it broke away from Sudan in 2011. In the rural Jonglei state, where the government is trying to put down a rebellion, the military itself faces charges of widespread abuses against the Murle ethnic group.