Thousands of youths loyal to South Sudanese rebel leader Riek Machar are marching on the strategic town of Bor, according to government sources.
Government troops are currently in control of Bor, the capital of Jonglei state they had taken from the rebels.
The government has offered to cease hostilities, but an army spokesman told the BBC that its forces were still battling over oilfields in the north.
At least 1,000 people have died since fighting broke out earlier this month.
More than 121,600 are believed to have fled their homes.
What began as a power struggle between Mr Machar and President Salva Kiir has taken on overtones of a tribal conflict. The Dinka, to which Mr Kiir belongs, are pitted against the Nuer, from which Mr Machar hails.
Tens of thousands of civilians have sought refuge in UN camps and reinforcements have been arriving to give them extra protection.
Joe Contreras, a spokesman for the UN Mission in South Sudan, told the BBC World Service Newshour programme that the reported advance of the armed youths was a dangerous development.
"One of the reasons why it is such a worrying development... is that they do not have a military background or the discipline that you would associate with military who have been fighting under the banner of the former vice-president since this crisis began," he said.
"So in effect they're kind of a wild card factor, though their sympathies appear to lie with the former vice-president."
He added there was no evidence they were acting on direct orders from Mr Machar.
The youths now believed to be marching on the town are said to be part of an ethnic Nuer militia known as the White Army, because of the white ash they put on their skin to protect them from insects.
Information Minister Michael Makuei Lueth earlier told the Associated Press that an estimated 25,000 people from the same tribe as Mr Machar were taking part.
"He has decided to mobilise the youth in the name of his tribe," Mr Lueth was quoted as saying.
But Moses Ruai Lat, a spokesman for the rebels, told AFP news agency this was not the case, but that they were simply regular soldiers turning their back on the government.
The BBC's James Copnall in Juba says it is unclear exactly how many people are heading towards Bor and who they are.
The UN is planning aerial reconnaissance flights on Sunday and the results of their search are vitally important, our correspondent adds.
Mr Machar was deputy president until Mr Kiir sacked him in July.
Earlier this month fighting broke out between rival army factions after Mr Kiir accused Mr Machar of trying to unseat him in a coup.
Mr Machar told the BBC on Friday that his forces were in control of the whole of the states of Jonglei and Unity, apart from the town of Bor.
He said he had a negotiating team ready but any ceasefire had to be serious, credible and properly monitored.
He called for the release of all 11 detainees accused of being co-conspirators in the coup plan, a key rebel condition for any negotiations.
Mr Kiir's spokesman Ateny Wek Ateng told the BBC that Mr Kiir would not accept any preconditions for a ceasefire.
"If Machar said he will not cease the hostilities, then he will of course be attacking our positions and our soldiers have the right to defend themselves," Mr Wek Ateng said.
He added that Mr Kiir had tried to avoid the conflict taking over tribal overtones, and blamed Mr Machar for the increased tension between the two ethnic groups.