"Reports from the UN International Organization on Migration (IOM) suggest that Government forces are now approaching the town of Maiwut, 25 kilometers northwest of Pagak," a town on the Ethiopian border, the report stated on Wednesday.
Citing IOM reports of South Sudanese crossing the border near Pagak, UN Special Representative in South Sudan David Shearer told reporters from the capital of Juba that he was "gravely concerned by this ongoing situation."
Shearer added that fighting had forced at least 25 aid workers near Miawut to relocate in the past week.
Nearly 1 million South Sudanese are currently sheltering in Uganda and another 200,000 have fled to other neighboring nations, according to UN estimates.
Shearer also expressed concern for 250 children in an orphanage south of Juba, which lies between government and rebel forces that are preparing to fight.
South Sudan achieved independence in 2011. Two years later, an ethnic-based civil war erupted when President President Salva Kiir of the Dinka tribe accused Vice President Riek Machar, an ethnic Nuer, of planning a coup.
Both sides have been accused of multiple atrocities.