The UN has expressed concerns about thousands of South Sudanese youths – loyal to rebel leader Riek Machar – marching on the strategic town of Bor.
Armed with machetes and sticks, the “wildcard” group does not have military training, a UN spokesman told the BBC.
The UN is organising surveillance flights to ascertain the group’s size, added the spokesman, Joe Contreras.
At least 1,000 people have died in this month’s fighting. More than 121,600 are believed to have fled their homes.
Tens of thousands of civilians have sought refuge in UN camps and reinforcements have been arriving to give them extra protection.
The government has offered a ceasefire, but the army says its forces are still battling over oilfields in the north.
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.
Government troops are currently in control of Bor, the capital of Jonglei state they had taken from the rebels.
The group reportedly marching on the town are 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.
South Sudanese government spokesmen have been quoted as saying it numbers as many as 25,000 armed men and answers to the former vice-president, but these details have not been confirmed.
The White Army seems sympathetic to Mr Machar, but does not appear to be acting on his direct orders, said Joe Contreras, a spokesman for the UN Mission in South Sudan, who described the group as “a volatile and unpredictable ingredient” to the unrest in South Sudan.
“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 told the BBC’s World Service.
“They are a wildcard whose intervention in the theatre of conflict outside Bor could ratchet up the conflict even further and also put at even greater risk the lives of innocent civilians.”
Mr Machar was deputy president until Mr Kiir sacked him in July.
Riek Machar has given a cautious response to government proposals to end hostilities, as Peter Biles reports
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 said on Friday his forces were in control of the whole of the states of Jonglei and Unity, apart from Bor.
He said he had a negotiating team ready but any ceasefire had to be credible, properly monitored and preceded by the release of 11 detainees accused of being co-conspirators in the coup plan.
Mr Kiir has refused to accept any preconditions for a ceasefire.