Embassies and aid organizations in South Sudan were trying to evacuate staff from the capital, Juba, on Tuesday as a precarious calm settled over the city following several days of deadly clashes.
"Several hundred people have already been killed, including civilians seeking refuge. Some of the civilians killed were reportedly targeted based on their ethnicity," the U.N. special adviser on preventing genocide, Adama Dieng, said in a statement Tuesday.
South Sudan's government has said at least 272 have been killed, including 33 civilians, in fighting that broke out Thursday night with gunfire between opposing army forces that raised fears of a return to civil war.
President Salva Kiir and former rebel leader, First Vice President Riek Machar, declared separate cease-fires Monday night. Military trucks drove up and down Juba's roads with megaphones Tuesday, ordering soldiers back to barracks.
It is "hugely worrying" that the fighting appeared to be spreading outside the capital, the U.N. human rights office in Geneva said Tuesday.
The U.S. Embassy, Doctors Without Borders and the International Medical Corps were among organizations pulling out their staffs from South Sudan. Private chartered planes flew foreigners out of Juba's reopened airport Tuesday, as regional carriers including Kenya Airways had cancelled flights there.
Japan dispatched military aircraft to evacuate its citizens. Uganda will send troops to Juba to evacuate its citizens, said Ugandan army spokesman Paddy Ankunda.
South Sudanese nationals trying to escape the capital were prevented from doing so by authorities, according to a security worker in Juba who insisted on anonymity because he is not authorized to speak to the press.
Aid groups warned about the lack of clean water for the tens of thousands of people sheltering in various sites around Juba as water tankers have not been able to make deliveries.
The fighting in Juba has severely threatened a peace deal signed last year between Kiir and Machar to end a civil war that had killed tens of thousands since late 2013. The agreement brought them and their supporters into a transitional coalition government in April.
In the latest fighting, government troops have lined up tanks and fired on a U.N. base where tens of thousands of civilians are sheltering, according to witnesses. At least eight civilians in the U.N. camp were killed in the crossfire.
Government officials have repeatedly accused the civilians inside the U.N. bases of being rebels or rebel supporters.
Two Chinese peacekeepers with the U.N. mission were also killed.
The government also overran one of Machar's two bases in Juba, deploying helicopter gunships and tanks against opposition forces carrying only light arms. Some 35 of Machar's bodyguards were killed in the latest clashes, said opposition military spokesman William Gatjiath.
South Sudan's civil war exposed deep ethnic fault lines in the country, pitting the Dinka supporters of Kiir against the Nuer followers of Machar.
The latest fighting has awakened fears that South Sudan's other ethnic groups will be drawn in.