There are conflicting reports from South Sudan where youths loyal to rebel leader Riek Machar are said to be marching on the strategic town of Bor.
In an interview with the BBC, a spokesman for President Salva Kiir denied earlier reports that most of the youths had been persuaded to go home.
Instead, Ateny Wek Ateng said the group had clashed with government forces.
At least 1,000 people have died in this month's fighting. More than 121,600 are believed to have fled their homes.
A UN surveillance flight earlier located the group of youths 50km (30 miles) north-east of Bor, but did not reveal its size.
Tens of thousands of civilians have sought refuge in UN camps, and reinforcements have been arriving to give them extra protection.
What began as a power struggle between Mr Machar and President Salva Kiir has taken on overtones of an ethnic conflict. The Dinka, to which Mr Kiir belongs, are pitted against the Nuer, from which Mr Machar hails.
The government has offered a ceasefire, but the army says its forces are still battling rebels over oilfields in the north.
UK Foreign Secretary William Hague said he had contacted President Kiir and Mr Machar "to urge them both to commit to an immediate ceasefire".
Mr Hague said he had encouraged them "to enter into negotiations immediately and without preconditions" and offered the UK's diplomatic support.
East African mediators have given both sides until Tuesday to agree a ceasefire.
South Sudanese government troops are currently in control of Bor, the capital of Jonglei state they took from the rebels.
The group said to be 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 were quoted as saying the group numbered as many as 25,000 armed men and answers to the former vice-president, but these details have not been confirmed.
Information Minister Michael Makuei Lueth told the BBC on Sunday that Nuer community leaders in Jonglei state had persuaded the fighters to go home. He said only a "small group" remained.
However, presidential spokesman Ateny Wek Ateng later denied this claim and said clashes had taken place.
He said the youths had "ignored" calls by community leaders to abandon their march.
"They seem to be adamant because they think that if they don't come and fight, then the pride of their tribe has been put in great insult," he said.
"The majority are still moving... there's still a number of about 20,000 still moving."
He added that government forces had been deployed to Bor.
Joe Contreras, a spokesman for the UN Mission in South Sudan, described the group of youths as "a volatile and unpredictable ingredient" to the unrest in South Sudan.
"They are a wildcard whose intervention in the theatre of conflict outside Bor could ratchet up the conflict even further."
Mr Machar was deputy president until Mr Kiir sacked him in July.
Earlier this month fighting broke out between rival armed 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.