The army in South Sudan has told all civilians to leave the city of Bentiu, saying it is about to recapture the oil hub from rebel forces.
Military spokesman Philip Aguer told the BBC government forces were on the edge of the city, with only a bridge separating them from the centre.
The rebels have not commented.
UN peacekeeping chief Herve Ladsous says the conflict has killed "very substantially in excess" of 1,000 people since 15 December.
Ceasefire talks in neighbouring Ethiopia have stalled.
Thousands of people have already fled Bentiu, one of two main cities seized by rebel forces.
Several thousand have sought refuge in a UN base in the city, where people have been divided according to their ethnic group in order to prevent clashes between them.
The conflict has seen outbreaks of ethnic slaughter betweens Dinkas, the community of President Salva Kiir, and Nuers, like rebel leader Riek Machar.
Col Aguer said a single rebel tank was resisting at the bridge but was confident the city would fall soon.
He said all civilians should leave the city to avoid being caught in the crossfire.
"The earlier they leave, the better," he told the BBC's Outside Source programme.
He did not give any casualty figures but said that fighters on both sides had been killed.
However, the UN envoy in South Sudan, Toby Lanzer, earlier tweeted from Bentiu that the city was quiet.
He said UN peacekeepers had built up defences at the UN base to protect civilians.
Bentiu is the capital of the oil-rich Unity state. The fighting has seen South Sudan's oil output fall by about 20%.
Swimming under gunfire
Col Aguer said that from Friday afternoon, all the government forces would be focused on recapturing Bor - the other city under rebel control.
On Thursday, people fleeing Bor told the AFP news agency that gunmen had shot dead fleeing civilians, torched entire villages and looted crops.
One cattle herder told of swimming across the River Nile while being shot at.
"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.
"The bullets were hitting the water, but we knew we could not stop or they'd shoot us."
Mr Ladsous on Thursday night told the UN Security Council that more than 250,000 had fled their homes because of the conflict.
South Sudan is the world's newest state. It became independent in 2011 after seceding from Sudan.