South Sudan's army is advancing on the key rebel-held centres of Bentiu and Bor, as rebels strengthen defences in Bentiu.
An army spokesman was quoted as saying that government troops were "next to Bentiu" and some 15km (9 miles) away from Bor. Reports say hundreds have fled violence in Bor.
At least 1,000 people have been killed in fighting since 15 December.
Regionally brokered talks to declare a ceasefire have stalled.
People who escaped the violence told AFP news agency that gunmen shot dead fleeing civilians, torched entire villages and looted crops.