JUBA, South Sudan - 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 preconditions 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 shelter at U.N. 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 last week, said a military spokesman, Col. Philip Aguer. Pro-Machar forces are believed to be preparing a fresh offensive to retake Bor, where three U.S. military aircraft were hit by gunfire while trying to evacuate American citizens Dec. 21, wounding four U.S. service members.

The "White Army" said to be marching on Bor gets its name from the ash fighters put on their skin as protection from insects.