Thousands of South Sudanese youths loyal to rebel leader Riek Machar are reported to be marching on the town of Bor, the capital of Jonglei state.
Government troops are in control of Bor after taking it from the rebels this week.
The BBC reports those marching on the town are part of an ethnic Nuer militia known as the White Army, because of white ash they put on their skin.
Mr Machar was deputy president until President Salva Kiir sacked him in July.
The United Nations mission in South Sudan says it will use reconnaissance planes to try to verify the number of supporters of Riek Machar reported to be marching toward Bor.
The BBC reports their power struggle has taken on overtones of a tribal conflict. Mr Machar hails from the Nuer tribe. Mr Kiir belongs to the Dinka.
Fighting broke out between rival army factions on 15 December after Mr Kiir accused Mr Machar of trying to unseat him in a coup.
The government has offered to cease hostilities, but an army spokesman said there is still fighting over oilfields in the north.
The UN Security Council approved plans on Tuesday to raise the number of UN peacekeepers in South Sudan to 12,500 troops and 1323 police.