“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.”