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, President Salva Kiir’s spokesperson Ateny Wek Ateng, denied reports that most of the youth had been persuaded to go home. Ateng said the group had clashed with government forces.At least 1,000 people have died in this month’s fighting while over 121,600 are believed to have fled their homes.A UN surveillance flight earlier located the group of youths 50 km 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 Machar and President Salva Kiir has taken on overtones of a tribal conflict. The Dinka, to which Kiir belongs, are pitted against the Nuer, from which 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 Machar and urged them to cease fire.Hague said he had encouraged them "to enter into negotiations immediately” and offered UK’s diplomatic support. East African mediators have given both sides until Tuesday to agree on a ceasefire.‘Volatile’South Sudanese government troops are currently in control of Bor, the capital of Jonglei state captured 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 authorities said the group numbered to 25,000 armed men loyal to the former vice-president, but these details have not been confirmed.Joe Contreras, a spokesman for the UN Mission in South Sudan, described the group as "a volatile and unpredictable ingredient” to the unrest in South Sudan.Earlier this month fighting broke out between rival armed factions after President Kiir accused Machar of trying to unseat him in a coup.Machar said on Friday his forces were in control of Jonglei and Unity. 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.