JUBA (AFP) - South Sudan's warring parties were on Wednesday due to start talks aimed at ending more than two weeks of bloodshed but rebel leader Riek Machar has warned his forces will keep fighting until a ceasefire deal is reached.
Machar has rejected face-to-face talks with his rival, President Salva Kiir, but has sent representatives to the talks in the Ethiopian capital as concerns soar over atrocities being committed in the world's youngest nation.
UN humanitarian officials say thousands are feared dead in the conflict which erupted on Dec 15 when Kiir accused Machar - who he sacked as his deputy in July - of mounting a coup against him.
Fighting quickly took on an ethnic nature, pitting Kiir's Dinka tribe against Machar's Nuer.
The UN Mission in South Sudan (UNMISS) said that "atrocities are continuing to occur" across the country despite efforts to negotiate a ceasefire.
"UNMISS is gravely concerned about mounting evidence of gross violations of international human rights law that have occurred in South Sudan during the past 15 days," it said in a statement.
Machar told AFP via satellite phone from an unknown location inside South Sudan that his forces were marching on the capital Juba.
"There is no cessation of hostilities yet," Machar said, adding any halt in hostilities needed to be negotiated.
"That is what the delegation going to Addis Ababa is going to discuss and to negotiate.
"I will follow later, once the negotiations have resulted in a cessation of hostilities. It depends on if and when that is achieved," Machar said.
His fighters, which include a feared Nuer militia called the "White Army", claimed Tuesday to have captured Bor, capital of the powder-keg Jonglei state and situated just 200km north of Juba - the third time the town has changed hands in two weeks.
"Bor is under our control... we are now in Bor town," rebel spokesman Moses Ruai told AFP.
Thousands had fled the town in anticipation of the attack, according to the UN and aid organisations.
"We did not ask for this battle, it was forced upon us," Machar added, reiterating his position that it was the president who started the fighting on Dec 15.
South Sudanese army spokesman Philip Aguer disputed the claim that Bor had been seized, saying fighting was ongoing inside the town.
Kiir has described the war as "senseless", but ruled out power sharing with the rebels.
"What power sharing? It is not an option. This man has rebelled. If you want power, you don't rebel so that you are awarded with the power," Kiir said in an interview broadcast on the BBC Tuesday.
The Sudanese army meanwhile said it had recaptured several areas bordering South Sudan on Tuesday.
South Sudan's hard won independence from Khartoum in 2011 has been beset by poverty, corruption and ethnic tensions.
Thousands of people are feared to have been killed in the fighting, pitching army units loyal to Kiir against a loose alliance of ethnic militia forces and mutinous army commanders nominally headed by Machar.
The African Union expressed "Africa's dismay and disappointment that the continent's newest nation should descend so quickly into civil strife".
Across the country, the UN has estimated close to 200,000 people have been forced to flee their homes, of which 75,000 have sought protection from badly overstretched UN peacekeepers.
There have also been grim reports of massacres, rapes and killings, prompting the African Union to threaten "targeted sanctions" over the conflict.
East Africa's regional IGAD bloc headed by Ethiopia - which had set Tuesday as the deadline for talks to start - said that negotiations "will focus on a monitored ceasefire" before more talks to settle "underlying political problems".