imageJUBA: South Sudan rebel leader Riek Machar agreed to send envoys to peace negotiations in Ethiopia Tuesday, but rejected face-to-face talks with President Salva Kiir, warning that his forces would continue to fight.

As the United Nations condemned “in the strongest possible terms" atrocities committed against innocent civilians, former vice president Machar said his troops were still marching on the capital after claiming to have recaptured a key town.

“Our forces are still marching on Juba, there is no cessation of hostilities yet," Machar told AFP via satellite telephone from an undisclosed location inside South Sudan.

He is accused by his foes of sparking deadly conflict by attempting a coup over two weeks ago,

Ignoring a deadline from regional powers for an immediate ceasefire, Machar said any halt in the more than two weeks of fighting “needed to be negotiated".

“That is what the delegation going to Addis Ababa is going to discuss and to negotiate," he said, adding the chance of him meeting with Kiir in person “depends on how the negotiations go".

The United States, which was a key backer of South Sudan's independence struggle, said the sending of negotiators was an “important first step".

Officials in the Ethiopian capital confirmed that delegations from both sides were due to land in Addis Ababa, the headquarters of the African Union, later Tuesday with talks expected to start on Wednesday.

“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.

“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 December 15.

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.

The rebel Sudan People's Liberation Army-North (SPLA-N), which has been fighting government troops in border states since 2011, denied the army's claim of gains in South Kordofan.

An armed forces spokesman quoted by state media said that troops had penetrated the Al-Ardiba area, a region whose non-Arab population's civil war sympathies largely lay with the now independent south.

Troops suffered several dead and wounded in the operation but seized “more than 30" military vehicles, the statement said.

South Sudan is the world's youngest nation, having only won independence from Khartoum in 2011, but has been beset by poverty, corruption and ethnic tensions, including between the president's Dinka tribe the largest in the country and Machar's Nuer community.

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 AU expressed “Africa's dismay and disappointment that the continent's newest nation should descend so quickly into civil strife".

Heavy fighting continued to rage on Tuesday, with the rebels claiming they had recaptured Bor, capital of the powder-keg Jonglei state and situated just 200 kilometres (125 miles) north of the capital 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.

South Sudanese army spokesman Philip Aguer disputed the claim, saying the fighting was ongoing inside the town. A UN spokesman had also confirmed the town was under attack earlier Tuesday.

'Full-fledged civil war'

Thousands have fled in recent days from Bor in fear of a counter-attack by rebels including an ethnic militia force dubbed the “White Army".

The UN Mission in South Sudan (UNMISS) said Tuesday it had counted “large numbers" of bodies of prisoners and civilians killed in the worsening conflict, with “atrocities continuing to occur" across the country.

“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," said an UNMISS statement.

“Extra-judicial killings of civilians and captured soldiers have occurred in various parts of the country, as evidenced by the discovery of large numbers of bodies" in the capital Juba, as well as Malakal and Bor, the main cities in Upper Nile and Jonglei states.

“I condemn in the strongest possible terms the atrocities committed against innocent civilians of different communities by elements from both sides during the crisis," said Hilde Johnson, UN representative in South Sudan.

Across the country, the United Nations 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."