JUBA, South Sudan (AP) — Twenty-five thousand young men who make up a tribal militia known as the "White Army" are marching toward a contested state capital in South Sudan, an official said Saturday, dimming hopes for a cease-fire.

Seeking an end to the nearly 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, accused by the government of leading a coup attempt on Dec. 15 that erupted into spiraling violence.

But Machar rejected that, saying in an interview with the BBC that any cease-fire had to be negotiated by delegations from 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 cease-fire 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 United Nations 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 this week, said military spokesman Col. Philip Aguer. Pro-Machar forces are believed to be preparing a fresh offensive to retake Bor, the Jonglei state town where three United States military aircraft were hit by gunfire while trying to evacuate American citizens on Dec. 21, wounding four U.S. 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 mobilize 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."

As of Saturday evening, the youths, who are armed with light weapons and heavy machine guns, were about 30 miles outside Bor, he said, meaning they could reach the state capital imminently.

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

Akshaya Kumar, a South Sudan analyst for the U.S.-based Enough Project, said it was important to remember that civilian lives hang in the balance in Bor.

"Bor has already been the site of two violent clashes in less than two weeks. Its people, many of whom are sheltering in the U.N. compound, cannot withstand another battle," she said. "The recent Lou Nuer storming of the U.N. base in Akobo set a dangerous precedent. We worry that the Bor peacekeeping force may not be able to withstand a similar onslaught."

South Sudan military forces are in Bor and will protect the civilian population against attacks, Lueth said. Most of the residents of Bor are Dinka.

"It's hard to predict what will happen," Lueth said. "This is war."

The White Army has threatened the central government in 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 U.S.-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 Machar, said 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 Machar, but there was no sign on Saturday 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 Machar for plotting a coup attempt on Dec. 15. 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 Machar's Nuer ethnic group. From Juba the military clashes then spiraled across the country.

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.

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Associated Press reporter Rodney Muhumuza in Kampala, Uganda, contributed to this report.