Talks between South Sudan's government and rebels are due to start in the near future in Ethiopia, say observers.
However there has been no immediate halt to hostilities, with reports that heavy fighting is continuing.
The conflict erupted more than two weeks ago between supporters of President Salva Kiir and those of his sacked deputy Riek Machar.
At least 1,000 people have died and more than 121,600 are believed to have fled their homes.
East African leaders have been leading mediation efforts to end the crisis.
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.
It appears pressure on Mr Machar has worked, says the BBC's James Copnall in the capital Juba, as he says he is sending a delegation to the talks despite Mr Kiir failing to agree to demands including the release from jail of Mr Machar's political allies.
But Mr Machar insists he will not order his troops to stop fighting before preliminary talks have taken place.
The end-of-Tuesday deadline was also missed, with Ethiopia now saying they hope talks in Addis Ababa will start on Wednesday and the UN's special representative to South Sudan, Hilde Johnson, saying they will begin on Thursday for "logistical reasons".
Mr Machar, who was deputy president until he was sacked in July, denies there was a plot to overthrow Mr Kiir.
The fighting initially broke out in Juba, and has now spread to many parts of the country.
The situation in the key town of Bor is fast-moving, but a government minister confirmed that the town had fallen to Mr Machar's forces.
Thousands of people are fleeing to the town of Awerial from Bor and surrounding areas, said David Nash of the medical charity Medecins Sans Frontieres, with many sleeping out in the open and facing a lack of clean water and sanitation.
The Sudanese army meanwhile said it had recaptured several areas bordering South Sudan on Tuesday.
Having apparently captured Bor, Mr Machar told AFP news agency that his forces were "still marching on Juba".
He said hostilities would only cease, and he would only arrive for talks in person, if preliminary negotiations in Addis Ababa were satisfactory.
Mr Machar claimed his delegation to talks would be headed by Rebecca Nyandeng, the widow of John Garang, who led South Sudanese rebel forces against Khartoum for many years - though she later denied this in a BBC interview.
As a Dinka, Ms Nyandeng would 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 and insect repellent.
Observers say the talks are likely to be complicated, as the two sides will have to agree on a mechanism to monitor a ceasefire.
Mr Kiir has also ruled out any power-sharing arrangement with Mr Machar in the longer term.
South Sudan is the world's newest state. It was formed in 2011, gaining independence from Sudan after decades of conflict.