An atmosphere of fear and desperation exist in South Sudan as violence escalates, a United Nations official has said.
Humanitarian Co-ordinator Toby Lanzer told the BBC there were summary executions in Bor, in the restive state of Jonglei that has fallen to rebels.
Clashes broke out between troops loyal to President Salva Kiir and others backing his former deputy a week ago.
Meanwhile, the United States said it had evacuated its citizens from Bor.
Earlier the South Sudanese army confirmed that Bentiu, the capital of oil-rich Unity State, had also fallen to troops supporting former Vice-President Riek Machar, the report said.
President Salva Kiir has accused Machar of attempting a coup.
According to the report, Lanzer added that there was a danger not just from fighting by conventional armies but from groups of youths who he said were simply 'out of control'.
Col Aguer said that Bentiu is in the hands of a commander who has declared support for Machar, adding that the number of people or wounded in the fighting was unclear, the report added.