JUBA, South Sudan — Rebel forces attacked the South Sudanese city of Bor on Tuesday, engaging in fierce fighting with government forces over the strategic position seen as a gateway to Juba, the capital of this young country.

The rebels controlled the city early in the revolt but were dislodged last week. They returned in stronger numbers, with renegade military units backed by the armed youth known as the White Army.

"It began this morning at around 7," said Col. Philip Aguer, a South Sudanese military spokesman. "It's heavy. It's very intense fighting."

For several days, groups including the United Nations warned that large numbers of fighters were advancing toward Bor, the capital of Jonglei State.

Regional leaders last week gave a Tuesday deadline for President Salva Kiir and his former vice president, Riek Machar, to begin peace talks. The fighting began after Mr. Kiir accused Mr. Machar, whom he fired in July, of an attempted coup. Mr. Machar has denied the charge.

On Thursday, the president of neighboring Uganda threatened to intervene if the rebels under Mr. Machar's command kept fighting, introducing the possibility of a broader regional conflict.

With fighting underway in Bor, last week's effort by East African leaders to push for a negotiated cease-fire seemed to have failed, at least for the time being. The question is how countries like Kenya, Ethiopia and Uganda will respond. 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.

More than 1,000 people have been killed since the clashes began on Dec. 15, 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 size of the White Army force, 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 been mobilized; others have put the number at a few thousand, saying that elders from their community had persuaded many to turn back.

"They are using big numbers we can't estimate," Col. Aguer said. "But the S.P.L.A. is committed to defend the area." The advance of the rebel forces sent civilians fleeing across the White Nile by the thousands as fighters burned homes in their path. The Nuer fighters were carrying AK-47s and had several heavy machine guns and 30 vehicles and trucks, according to the South Sudanese military.

Col. Aguer said that the S.P.L.A. had a brigade in and around Bor and more forces between Juba and Bor who could be sent in as reinforcements.

Bor appeared to be the epicenter of the violence on Tuesday. 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.

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 are in rebel hands. The fight for Bor could turn particularly nasty, with neither side wanting to give up such a prize. The city is just a few hours' drive north of Juba.

"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."