rebel youths march on Bor 28 December 2013 Last updated at 21:03 Riek Machar has given a cautious response to government proposals to end hostilities, as Peter Biles reports Thousands of youths loyal to South Sudanese rebel leader Riek Machar are marching on the strategic town of Bor. 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 his forces were still battling over oilfields in the north. More than 121,600 have fled their homes and at least 1,000 people killed since fighting broke out earlier this month. 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. Wild card 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 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 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. No preconditions 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 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.