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.
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.
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.