Expectations of a major upsurge in fighting came as the United Nations warned that the situation in the world's youngest nation was fast unravelling, with hundreds of thousands of civilians now at risk.
Fighting has gripped South Sudan for more than a week, after President Salva Kiir accused his former deputy Riek Machar, who was sacked from the government in July, of attempting a coup.
Machar denied the claim and accused Kiir of carrying out a vicious purge of his rivals. Vowing to oust Kiir, his forces have since seized the town of Bor, capital of the powder keg eastern Jonglei state and located just 200 kilometres north of Juba, as well as the town of Bentiu, capital of crucial oil-producing Unity state.
South Sudan army spokesman Philip Aguer said the government was on the offensive.
"Our soldiers will regain control of Bor town from the forces of the rebellion. The forces of Machar are still in control of the town, but we are readying to take back control," he told AFP.
The comments came despite days of shuttle diplomacy by African nations and calls from the United States, Britain and the United Nations for the fighting to stop.
The UN's top humanitarian coordinator in South Sudan, Toby Lanzer, visited the besieged town of Bor on Sunday, and said the situation was rapidly deteriorating.
"It would have been have been difficult one week ago to imagine that things would have unravelled to this extent," Lanzer told AFP.
"There are hundreds of thousands of South Sudanese who've fled into the bush or back to their villages to get out of harm's way," he said.
Asked which areas of the conflict-torn country he was most concerned about, Lanzer said that "it would be quicker to talk about which areas I'm not worried about."
"I hope to be wrong, otherwise, hundreds of thousands will need help very soon," he said, adding he was "very concerned that a battle looms" in Bor, where he admitted that the UN peacekeepers were unlikely to be in a position to protect the estimated 15,000 civilians seeking shelter at the UN base there.
The clashes have left hundreds dead - possibly many more - and sent tens of thousands of people fleeing for protection in UN bases or to safer parts of the country, which only won independence from Sudan in 2011.
The young nation is oil-rich but deeply impoverished and awash with guns after the long war with Khartoum, and has grappled with corruption and lawlessness since independence.
There are both ethnic and political dimensions to the fighting, as troops loyal to Kiir, an ethnic Dinka, battle forces backing Machar, a Nuer.
Foreign governments, including in Britain, Kenya, Lebanon, Uganda and the US, have been evacuating their nationals.
On Saturday four US servicemen were wounded when their aircraft came under fire in a rebel-held area.
Britain is sending its third and final military aircraft on Monday to evacuate citizens, warning those who chose to stay "may have difficulty leaving in the event of a further deterioration in security".
