South Sudan's army is advancing against the key rebel-held centres of Bentiu and Bor, as rebels strengthen defences in Bentiu.
An army spokesperson was quoted as saying that government troops were "next to Bentiu" and some 15 kilometres from Bor. Reports say hundreds have fled from Bor to escape the fighting.
At least about 1,000 people have been killed in fighting since December 15.
Regionally brokered talks to declare a ceasefire have stalled.
People who escaped the violence told an AFP reporter in the area that men with machine guns shot dead fleeing civilians, and torched entire villages and looted crops.
A cattle herder who swam across a river to escape said he was fortunate to survive.
"They [the attackers] had a machine gun raised up on a sandbank, and they fired and fired and fired as we swam," Gabriel Bol told AFP.
Meanwhile, South Sudan's rebels are strengthening their defences in the key city of Bentiu in anticipation of a government offensive to recapture it.
The BBC reports from the city says rebels brought a tank into the city in the oil-rich area, as the front line moves closer.
