JUBA, South Sudan — As the South Sudanese military warned that a rebel column of armed youths had advanced toward the city of Bor on Monday, the president of neighboring Uganda threatened to intervene if the rebels kept fighting, introducing the possibility of a broader regional conflict.
Col. Philip Aguer, a South Sudanese military spokesman, said rebel forces known as the White Army were 18 miles from Bor on Monday afternoon and had fought skirmishes with government troops on Sunday. He said the advance of the rebel forces had sent civilians fleeing across the White Nile by the hundreds as fighters burned homes in their path.
In the battles that have gripped this young nation this month, Bor was briefly captured by rebels and then quickly retaken by the military, known as the Sudan People’s Liberation Army. “The S.P.L.A. is ready to defend the town and protect themselves,” Colonel Aguer said.
With troops marching on Bor, last week’s effort by East African leaders to push for a negotiated cease-fire to the conflict seemed to have failed, at least for the time being. President Yoweri Museveni of Uganda on Monday called on regional nations to intervene to “defeat” the rebel forces if they did not agree to a cease-fire.
Fighting began on Dec. 15 with clashes between soldiers from the Republican Guard. President Salva Kiir accused his former deputy, Riek Machar, of trying to mount a coup. Mr. Machar fled into hiding and has been demanding Mr. Kiir’s resignation. More than 1,000 people have been killed in the ensuing clashes, including large numbers of civilians, and close to 180,000 people have been displaced over the two weeks of conflict.
“We gave Riek Machar four days to respond, and if he doesn’t we shall have to go for him, all of us — that is what we agreed in Nairobi,” Mr. Museveni told reporters, referring to a meeting of East African leaders in the Kenyan capital last week.
That raised the prospect of an escalation or even cross-border spillover, adding to a worrying picture for a region already suffering from bloodshed in the Central African Republic and the Democratic Republic of Congo.
Mr. Museveni and Mr. Kiir have been close for years, and the Uganda People’s Defense Force provided significant support to the Sudan People’s Liberation Army during the civil war against the Sudanese government in Khartoum, analysts said.
“The U.P.D.F. has always been a very good friend of the S.P.L.A.,” said Mareike Schomerus, a researcher on South Sudan at the London School of Economics. “Some would say without U.P.D.F. the S.P.L.A. would never have been able to fight the war in that way.”
Mr. Machar has said Ugandan aircraft have bombed rebel positions, an assertion Uganda has denied.
“That remains speculative, and I have no idea that we’ve engaged in such an action at all,” said Lt. Col. Paddy Ankunda, a spokesman for the Ugandan military. “But our briefing is very clear. Should we be attacked, our soldiers have a right to defend themselves.”
There have been conflicting reports about the advance of the White Army, called that because of the white ash that fighters rub onto their skin. South Sudan’s information minister said last week that 25,000 Nuer youths had gathered; others have put the number at a few thousand, saying that elders from their community had persuaded many to turn back.
The United Nations confirmed through a helicopter surveillance flight Sunday that a group was marching toward Bor, saying in a statement that it was “extremely concerned” about the reports. Many of the fleeing civilians have crossed the river into a neighboring state. The Nuer fighters were carrying AK-47s and had several heavy machine guns and 30 vehicles and trucks, Colonel Aguer said.
South Sudanese officials said Mr. Machar controlled the White Army, which he has denied. “This is a group of loyalists to Riek Machar,” Colonel Aguer said. He added that Malakal, the capital of Upper Nile State, where the two sides have clashed in recent days, was calm and under the control of the military. Bentiu, the capital of Unity State, was also peaceful but under rebel control, he said.
The question is what kind of casualties might occur if the South Sudanese Army, alone or with assistance, tried to retake places like Bentiu that lie in rebel hands.
“Regional armies need to assure that their use of force stays firmly within international law, and that civilians are under no circumstances targeted,” Ms. Schomerus said. “It is right now unclear which actors are committing what kind of atrocities — but what is already clear is that civilians are not being sufficiently protected by anybody, and quite possibly even targeted.”