On Tuesday, Machar told AFP via satellite phone that he was not yet ready to agree to an immediate ceasefire nor hold face-to-face talks with Kiir, and that his forces were marching on the capital Juba.
“There is no cessation of hostilities yet,” Machar said. “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.”
Kiir has described the war as “senseless”, but has ruled out power sharing with the rebels. The president has also rejected rebel demands that a number of their loyalists, arrested shortly after the violence started, be released.
“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.
On Wednesday the government confirmed it had lost control of Bor, a town which has changed hands three times in the past two weeks, but officials said fighting was continuing on a number of fronts.
South Sudan is the world’s youngest nation, having won independence from Sudan in 2011 after decades of civil war.
Industry sources say South Sudan’s oil production – most of which is bought by China – has dropped by around a fifth because of the fighting.
The United States, which was a key backer of South Sudan’s independence struggle, has warned of a “very complicated, tenuous situation”.
The AU expressed “Africa’s dismay and disappointment that the continent’s newest nation should descend so quickly into civil strife”, warning of its potential to deteriorate into “full-fledged civil war” – even though many observers say this has already happened.
A group of Ugandan lawmakers meanwhile accused President Yoweri Museveni of meddling in neighbouring South Sudan.
On December 20, Kampala deployed soldiers to South Sudan to evacuate stranded Ugandan nationals.
The MPs objected to a warning by Museveni for the rebels to stand down or face regional action.
“This issue of President Museveni giving ultimatums to Machar is not good. This is a tribal fight and not a political matter that he should get involved in,” Theodore Ssekikuubo told AFP.