South Sudan's warring parties were Wednesday set to begin peace talks in Addis Ababa aimed at bringing an end to a nearly three-week-old civil war that has already left thousands dead. Government and rebel negotiating teams both arrived in the Ethiopian capital by Wednesday evening, a source close to the talks said and diplomats said they expected formal discussions on a possible ceasefire to begin on Thursday, although informal contacts may take place later Wednesday. Ethiopian government spokesman Getachew Reda said the talks would focus on "monitoring mechanisms for the ceasefire". "It's positive that they are sending delegations," UN special envoy Hilde Johnson said in Juba, underscoring the dire need for "reconciliation and healing" at the talks, which are being brokered by Ethiopia on behalf of IGAD, an East African regional grouping. "Negotiations also need to be accompanied by something else, a deeper process that focuses on national reconciliation between the communities. We have seen terrible acts of violence in the past two weeks... and as we know, if there is no one held accountable, there is a major risk that the violence can continue," she added. Fighting erupted in South Sudan December 15, when President Salva Kiir accused his former deputy Riek Machar of attempting a coup. Machar has denied this, in turn accusing the president of conducting a violent purge of his opponents. The fighting has spread across the country, with the rebels seizing several areas in the oil-rich north. On Tuesday the rebels recaptured Bor, the capital of Jonglei state and situated just 200 kilometres (125 miles) north of the capital Juba, and fighting was reportedly continuing in the area on Wednesday. Thousands of people are feared dead, UN officials say, while close to 200,000 civilians have been forced to flee their homes -- many seeking refuge with badly overstretched UN peacekeepers. The International Committee of the Red Cross said residents continue to pour by the thousand out of Jonglei and across the White Nile into Awerial county in neighbouring Lakes state. "The road to the river is lined with thousands of people, with others waiting for boats to carry them across," said Francois Moreillon, ICRC deputy head of delegation. The UN last week estimated that some 70,000 people had already fled into Awerial. The conflict has also been marked by an upsurge of ethnic violence pitting members of Kiir's Dinka tribe against Machar's Nuer community, and the UN Mission in South Sudan (UNMISS) said that "atrocities are 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," it said in a statement, reporting "extra-judicial killings of civilians and captured soldiers" and the "discovery of large numbers of bodies" in Juba, Bor and Malakal, the main town in oil-producing Upper Nile state. UN gathering evidence on atrocities The UN mission also said it was "actively collecting information" on the atrocities to be used for future official investigations. 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.