Bentiu has seen fierce battles over the last 24 hours, said the aid group CARE, which added that its staff is taking shelter in bunkers on the UN base.
The rebels launched a counter-offensive Monday and the city may have changed hands yet again, according to a security expert, who insisted on anonymity because he is not authorized to speak to the press.
A government offensive yesterday came just days after South Sudanese President Salva Kiir told US Secretary of State John Kerry that he was ready to hold peace talks with the rebel leader, former Vice President Riek Machar.
"The Americans are pushing us to go to Juba and form an interim government. We cannot go there without an agreement on a program first. We need to know who will be in that transitional government, in what capacity, for how long and issues like that," said the spokesman, Yohanis Musa Pouk.