Twenty-five thousand young men who make up a tribal militia known as the "White Army" are marching towards a contested state capital in South Sudan, an official has said, reducing hopes for a ceasefire.
Seeking an end to the two-week crisis in which an estimated 1,000 people have been killed, leaders from across East Africa announced on Friday that South Sudan had agreed to a "cessation of hostilities" against forces loyal to former vice president Riek Machar.
The government accuses Mr Machar of leading a coup attempt on December 15 that erupted into growing violence.
But Mr Machar rejected that, saying in an interview with the BBC that any ceasefire had to be negotiated by delegations from both sides. The government in the capital, Juba, seized on that statement to further condemn Mr Machar.
"Dr Riek Machar has put obstacles to this genuine call by issuing pre-conditions that a ceasefire cannot be reached unless a negotiation is conducted," said vice president James Wani Igga. "This is complete intransigence and obstinacy because the main issue now is to stop violence."
In addition to those killed, tens of thousands are seeking shelters at UN camps. More fighting is expected.
There is a looming battle for Bor, the provincial capital of Jonglei state that briefly fell to rebels before government forces took it back this week, said military spokesman Col Philip Aguer.
Pro-Machar forces are believed to be preparing a fresh offensive to retake Bor, he said. Bor is the town where three United States military aircraft were hit by gunfire while trying to evacuate American citizens on December 21, wounding four US service members.
An estimated 25,000 youths from the Lou Nuer sub-clan - the same tribe Mr Machar is from - are marching on Bor, said information minister Michael Makuei Lueth.
The "White Army" gets its name in part from the white ash fighters put on their skin as a form of protection from insects.
"He has decided to mobilise the youth in the name of his tribe," Mr Lueth said.
The White Army has threatened the central government in the recent past. In 2011 the army said that the Nuer youths would fight until all the Murle - another tribe - had been killed. The statement warned the national military to stay out of the way.
Another statement warned that the White Army would "wipe out" the army, according to the Enough Project, a US-based advocacy group that works on issues in central Africa.
Elsewhere, in oil-rich Unity state government troops were being forced to repel attacks by forces loyal to Mr Machar, said Col Aguer. The military "is fighting back, but it is the other side that is attacking us", he said.
IGAD, the regional bloc of East African nations, demanded on Friday that negotiations begin before the end of the year between South Sudan's government and Mr Machar, but there was no sign that is likely.
"We are ready to meet even before that. It is now up to Machar to accept the ceasefire," said vice president Igga.
The government blames Mr Machar for plotting a coup attempt on December 15.
Mr Machar denies that charge and his backers insist violence began when presidential guards from President Salva Kiir's majority Dinka tribe tried to disarm guards from Mr Machar's Nuer ethnic group.
The United Nations, South Sudan's government and other analysts say the dispute is political at its heart, but has since taken on ethnic overtones. The fighting has displaced more than 120,000 people.