Thousands fled the flash-point town of Bor as South Sudanese troops prepared for a possible assault by anti-government forces, including an ethnic militia called the “White Army” known for its brutality, said South Sudanese officials and aid workers on Monday.

“We are expecting an attack at any moment,” said Col. Philip Aguer, a spokesman for the South Sudanese military, adding that the militia was thought to be 18 miles outside Bor. “The civilian population has already left the town. If there are any left, they are hiding at the U.N. base.”

The warnings of the impending assault suggest that the conflict dividing the world’s newest country could intensify, even as American and African mediators have pressed the government and rebels to enter a cease-fire and negotiate a peace deal. The conflict began two weeks ago after President Salva Kiir accused his deputy, Riek Machar, of attempting a coup, triggering widespread fighting between soldiers loyal to each of the men. Hundreds of civilians have been killed; and at least 180,000 people have fled their homes, including about 75,000 who are taking refuge in U.N. bases nationwide, according to the United Nations.

The political struggle has sparked a tribal conflict, pitting ethnic Dinkas, like Kiir, against ethnic Nuer, Machar’s group. The White Army is a gang of Nuer youths loyal to Machar and numbering in the thousands. They are called the White Army because of the white ash, made from dried cow dung, they apply on their bodies, apparently to shield them from insects.

“They have started burning down villages 25 miles outside of Bor,” Aguer said.

The assertions could not be independently verified. There have been conflicting reports of the White Army’s march toward Bor, which is 120 miles north of Juba, the capital. Some reports say community leaders have persuaded the youths to return home, while others say government forces have clashed with the White Army outside Bor.

On Monday, the violence threatened to turn into a regional conflict, as Ugandan President Yoweri Museveni arrived in Juba to demonstrate support for the government of Kiir, his close ally. Muse­veni warned that South Sudan’s neighbors would target Machar if he didn’t agree to a cease-fire proposed by East African countries by the Tuesday deadline.

“We gave Riek Machar four days to respond, and if he doesn’t, we shall have to go for him, all of us,” Museveni told reporters, according to the Reuters news agency. When asked what that meant, he replied, “To defeat him,” but he did not provide details.

Machar has denied that he plotted a coup, but he has launched a rebellion and seized key towns, including Bentiu, the capital of oil-rich Unity state. He and his loyalists also have said it was not true that he controlled the White Army. But his rivals insist that he is the group’s benefactor.

“He is the one who organized and agitated them,” said South Sudan’s minister of information, Michael Makuei.

Makuei added that many civilians are fleeing the rural areas surrounding Bor, fearing the advance of the White Army and soldiers loyal to Machar.

“People are moving in the thousands,” Makuei said. “They are going in any direction that takes them to safety. Some are fleeing down the river, using motor boats and canoes, while others are traveling by land.”

On Monday, an international aid agency, Doctors Without Borders, said in a statement that more than 70,000 people, mostly women and children, had fled Bor. Most had crossed the White Nile river into Lakes state and were gathered around the town of Awerial, about 30 miles from Bor, it said.

South Sudanese officials said government forces have been bolstered to protect Bor and prevent the White Army or Machar loyalists from marching toward the capital.

“The fighting will not reach Juba,” Makuei said.