Twenty-five thousand young men from South Sudan’s white army tribal militia are marching toward a contested state capital, dimming hopes for a ceasefire.

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, accused by the government of leading a coup attempt on December 15 has led to an estimated 1000 deaths.

But Machar told BBC any ceasefire had to be negotiated by both sides. The government in the capital, Juba, seized on that statement to further condemn 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. Most serious is the looming battle for Bor, the provincial capital of Jonglei state that briefly fell to rebels before government forces took it back, said military spokesman Colonel Philip Aguer.

Pro-Machar forces are believed to be preparing a fresh offensive to retake Bor, the Jonglei state town where three US military aircraft were hit by gunfire while trying to evacuate American citizens on December 21, wounding four US service members.

The estimated 25,000 youths from the Lou Nuer sub-clan – the same tribe Machar is from – are marching on Bor, said Information Minister Michael Makuei Lueth.

  The white army gets its name from the white ash fighters put on their skin as protection from insects.

“He has decided to mobilise the youth in the name of his tribe,” Lueth said.

The estimate of 25,000 came from intelligence inside the group itself, Lueth said. Asked if the government was monitoring the group from the air, he said only: “Well, ultimately we are monitoring.”

The youths, who are armed with light weapons and heavy machine guns, were about 50km outside Bor, he said.

Earlier in the crisis some 2,000 Lou Nuer armed fighters attacked a UN base in Akobo, also in Jonglei state, killing three UN troops and a reported two dozen or so ethnic Dinka inside the base.