South Sudan President Salva Kiir has ruled out any power sharing with rebel leader Riek Machar to halt violence that has killed at least 1,000 people.
Mr Kiir told the BBC that Mr Machar - his deputy until sacked in July - should not be rewarded with power for rebelling.
The fighting broke out more than two weeks ago in the capital Juba. It has spread to many parts of the country.
South Sudan only became independent in 2011, after decades of conflict.
What began as a power struggle between the two men has taken on overtones of an ethnic conflict. The Dinka, to which Mr Kiir belongs, are pitted against the Nuer, from which Mr Machar hails.
In an interview with the BBC, President Kiir said Mr Machar had not earned the right to share the leadership of South Sudan.
Riek Machar (left) and Salva Kiir, shown in a photo from last July, are under pressure to negotiate
Sharing power, President Kiir said, "is not an option".
"These men have rebelled. If you want power, you don't rebel so that you are rewarded with the power. You go through the process."
He recalled that he did not come to power through a military coup but via the ballot box.
"Elections are coming in 2015 - why did he not wait so that he went through that same process?" he asked.
Mr Kiir also refused to release political allies of his rival who have been detained. Mr Machar has said he will not negotiate unless the men are freed.
Fighting has reportedly continued ahead of Tuesday's deadline set by regional leaders for peace talks to begin.
Ugandan President Yoweri Museveni is in South Sudan to try to help start negotiations.
The BBC's James Copnall says aid workers fear a humanitarian crisis
He has threatened military action against Riek Machar if he refuses to co-operate.
Meanwhile, on the ground, families have been split up as people fled their homes when the fighting started.
Thousands of children are likely to have been separated from their families and many children are surviving on their own in very remote areas, aid agency Save the Children says.
Some have witnessed their parents being killed and their homes looted or destroyed.
Fighting broke out between rival army factions some two weeks ago after allegations of a coup attempt
What began as a power struggle has taken on overtones of an ethnic conflict
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
Both Sudan and the South are reliant on oil revenue, which accounts for 98% of South Sudan's budget. They have fiercely disagreed over how to divide the oil wealth of the former united state - at one time production was shutdown for more than a year. Some 75% of the oil lies in the South but all the pipelines run north
The two Sudans are very different geographically. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
After gaining independence in 2011, South Sudan is the world's newest country - and one of its poorest. Figures from 2010 show some 69% of households now have access to clean water - up from 48% in 2006. However, just 2% of households have water on the premises.
Just 29% of children attend primary school in South Sudan - however this is also an improvement on the 16% recorded in 2006. About 32% of primary-age boys attend, while just 25% of girls do. Overall, 64% of children who begin primary school reach the last grade.
Almost 28% of children under the age of five in South Sudan are moderately or severely underweight - this compares with the 33% recorded in 2006. Unity state has the highest proportion of children suffering malnourishment (46%), while Central Equatoria has the lowest (17%).
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