Recent violence in South Sudan has pushed the country to the brink of a civil war, forcing tens of thousands to flee. This photo, taken Friday, shows the U.N. compound in the country. Ben Curtis/AP

As many as 25,000 young men who make up an armed tribal group 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-old 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 contends he lead a coup attempt Dec. 15 that erupted into violence.

But Machar rejected that notion, 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 and the U.N. has called the situation "tense."

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 Dec. 21. Four U.S. service members were wounded.

An estimated 25,000 youths belonging to a sub-clan of the Nuer tribe, which is Machar's tribe, are marching on Bor, said Information Minister Michael Makuei Lueth.

"He has decided to mobilize the youth in the name of his tribe," Lueth said.

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

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

The so-called White Army gets its name in part from the white ash fighters put on their skin as a form of protection from insects.

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" government forces, according to the Enough Project, a U.S.-based advocacy group that works on issues in central Africa.

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.

The Associated Press