South Sudan rebel chief Riek Machar on Tuesday promised to attend peace talks to end a brutal civil war in the world's newest nation as the US unveiled its first sanctions in response to the "unthinkable violence".
Washington's sanctions targeted one military leader from each side in the nearly five-month conflict that has continued to rage in defiance of international pressure and despite dire warnings of genocide and famine.
The two men were "responsible for perpetrating unthinkable violence against civilians," said US Secretary of State John Kerry.
The move against Marial Chanuong, also known as Marial Chinoum, a commander of the South Sudanese presidential forces, and Peter Gadet, a leader of the anti-government rebels, comes in the wake of a visit by Kerry to Juba last week where he called on both sides to lay down their arms.
Tuesday saw the arrival of UN Secretary General Ban Ki-moon in the South Sudan capital.
"The conflict destroying this fragile young country is senseless," Ban said, warning that if fighting continues one million people could soon face famine.
He confirmed that rebel leader Machar, a former vice president, "will be present" at peace negotiations in the Ethiopian capital.
During his visit, Kerry had extracted promises from President Salva Kiir that he would meet his nemesis for face-to-face talks.
Both sides have been accused of widespread ethnic massacres, rape and recruitment of thousands of child soldiers. Fierce fighting continues for control of a key oil town in the north of the country.
"What I have seen and heard today breaks my heart and enrages my soul," Ban told reporters, as he wrapped up his one-day visit, where he met Kiir as well as some of the tens of thousands sheltering inside hugely overcrowded fortified UN peacekeeper bases.
They are too frightened of attacks to leave the protection of the peacekeepers, even though the camps themselves have come under attack and conditions are dire with flooding and little shelter from alternating baking heat and torrential rains.
- 'Senseless' war rages -
Ban said the meeting between Kiir and Machar had been tentatively scheduled for Friday, although the date may be pushed back as Machar has said it will take time for him to travel to the venue.
Despite US warnings of sanctions if fighting continued, the government has pushed forward a major offensive to claw back towns from the rebels, capturing a opposition stronghold and forcing Machar to flee into the bush.
"Leaders must close the wound they have opened, they must support justice and accountability for crimes committed, and they must address the root causes of the conflict," Ban added.
Ban last visited South Sudan amid euphoric celebrations at its independence from Sudan in July 2011, after it voted to split away following decades of war with Khartoum.
But as he arrived Tuesday, battles raged around the dusty northern town of Bentiu, ramshackle capital of the oil-producing Unity state, four days after government troops moved to wrest back control.
Defence Minister Kuol Manyang told AFP government troops were in control of the town centre, but that heavy fighting was ongoing in surrounding villages.
Forces include both mutinous troops as well as ragtag forces, including a fearsome militia called the White Army, after the ash they daub on their bodies, both to ward off mosquitoes and as war paint.
The war has claimed thousands -- and possibly tens of thousands -- of lives, with over 1.2 million people forced to flee their homes.
Almost five million people are in need of aid, according to the UN.
A January ceasefire was never enforced. Stop-start peace talks in Ethiopia have yet to forge agreement on even the basic agenda.
Aid agencies are warning that South Sudan is now on the brink of Africa's worst famine since the 1980s, while both Kerry and the UN human rights chiefs have spoken out over their fears that the country could slide towards a genocide.
Although starting as a personal rivalry between Kiir and Machar, the conflict has seen armies divide along ethnic lines and fighting pitting members of Kiir's Dinka tribe against Machar's Nuer.
The conflict erupted on December 15 with Kiir accusing Machar of attempting a coup. Machar then fled to the bush to launch a rebellion, insisting that the president had attempted to carry out a bloody purge of his rivals.