The United Nations mission in South Sudan is withdrawing non-critical staff from the country as violence escalates.
The military has confirmed the capital of the oil-rich Unity State, Bentiu, has fallen to troops supporting former vice-president Riek Machar.
Clashes erupted between rival troops a week ago after President Salva Kiir accused Mr Machar of attempting a coup.
A UN official in South Sudan has told the BBC of summary killings on the streets and an atmosphere of fear and desperation.
UN humanitarian coordinator Toby Lanzer said as well as people seeking refuge in UN compounds there were many more hiding out in the bush.
Mr Lanzer added that there was a problem not just with fighting by conventional armies but with groups of youths who he said were simply "out of control".
The UN mission UNMISS said it had begun relocating staff from the capital Juba to the Ugandan city of Entebbe.
Meanwhile, the government says it is trying to retake the town of Bor in Jonglei State - another regional capital that has fallen to the rebels
