Talks between South Sudan’s government and rebels are due to start later on Tuesday, mediators say.
The two sides are expected to reach an agreement on the cessation of hostilities, they said.
The talks are the first since conflict erupted two weeks ago between forces loyal to President Salva Kiir and his sacked deputy, Riek Machar.
The meeting will take place in Ethiopia, following mediation efforts by East African leaders.
On Monday, Uganda’s President Yoweri Museveni threatened the rebels with military action if they failed to agree to a ceasefire by the end of Tuesday, and begin talks.
Representatives of Mr Kiir and Mr Machar would meet in the Ethiopian capital, Addis Ababa, to defuse tensions in South Sudan, the Ethiopian government said in a statement.
Regional leaders had wanted a cessation of hostilities and talks to begin by 31 December. The attack on Bor was a clear attempt by Mr Machar to show his military power, which will strengthen his hand in any negotiations.
He also said his delegation will be led by Rebecca Nyandeng, the widow of the South Sudanese hero John Garang. As a Dinka, she may help Mr Machar challenge the allegation that his rebellion is primarily from his Nuer ethnic group.
It is interesting that Mr Machar is now admitting that the “white army” – an ethnic militia – is “part of” his army. This will not do much for his popularity in many parts of South Sudan.
“The two sides are expected to reach an agreement on the cessation of hostilities and peaceful resolution of the current political crisis,” the statement added.
Earlier, Mr Machar told the BBC he would send a delegation to the talks, claiming his forces had captured the key town of Bor.
But while he had agreed to negotiate, he said he would not order his troops to stop fighting.
Mr Machar had previously demanded 11 detainees accused of being co-conspirators in a coup plan be freed before negotiations.
He denies there was a plot – alleged by Mr Kiir.
The fighting initially broke out more than two weeks ago in South Sudan’s capital, Juba, and has now spread to many parts of the country.
At least 1,000 people have died and more than 121,600 are believed to have fled their homes.
The situation in Bor is fast-moving, but a government minister confirmed that the town had fallen to Mr Machar’s forces, reports the the BBC’s James Copnall from Juba.
A UN spokesman said Bor, the capital of Jonglei state, had come under attack at day break, not far from the town’s UN compound.
Mr Machar said his delegation to talks will be headed by Rebecca Nyandeng, the widow of John Garang, who led South Sudanese rebel forces against Khartoum for many years.
Our reporter says as a Dinka she may help Mr Machar challenge the allegation that his rebellion is primarily from his Nuer ethnic group.
Mr Machar’s forces are a mix of mutinous soldiers loyal to him and an ethnic militia called the “white army”, known for putting white ash onto their bodies as a kind of war-paint.
Observers say the talks are likely to be complicated, as the two sides will have to agree on a mechanism to monitor a ceasefire.
South Sudan is the world’s newest state. It was formed in 2011, gaining independence from Sudan after decades of conflict.