Rebel forces, who retook control of the pivotal South Sudanese town of Bor this week, marched south toward the capital, Juba, on Thursday, clashing with government troops, even as the warring sides prepared to sit down for peace talks in neighboring Ethiopia.

A spokesman for the South Sudanese military, Col. Philip Aguer, said the rebel forces — who are loyal to former vice president Riek Machar — had again taken over Bor, the capital of Jonglei state. The rebels, made up of defected soldiers and an ethnic militia known as the “White Army,” then moved about 13 miles south of the town, along the road leading to Juba.

Fighting erupted Thursday afternoon between the rebels and the government’s Sudan People’s Liberation Army (SPLA) outside Bor, as well as in parts of the oil-producing Unity state.

“The other side is not interested in a cease-fire,” Aguer said. “They are the ones advancing, and the SPLA is fighting back defensively.”

The clash and the rebels’ apparent push toward Juba, which lies 120 miles from Bor, are likely to pose challenges to efforts by American and African mediators to negotiate an end to the conflict, which has entered its third week. Representatives of Machar and President Salva Kiir have arrived in the Ethiopian capital, Addis Ababa, and are expected to discuss a cease-fire and a peace deal.

Analysts say that any push toward Juba by the rebels could be an effort to apply more pressure on Kiir to step down, rather than a tactical move to seize the capital. In Juba, most of the SPLA is considered loyal to Kiir, and the rebels could find themselves in a precarious military position.

“If the rebels present enough of a military threat to the capital, people in the leadership may realize that the only way forward is some sort of change at the top,” said J. Peter Pham, director of the Atlantic Council’s Africa Center.

The crisis was triggered by a dispute, followed by fighting, between soldiers loyal to Kiir and those aligned with Machar on Dec. 15. That grew into allegations by Kiir that Machar was trying to seize power. Within days, the fighting had spread to more than 20 towns across the fragile oil-producing nation. Hundreds of people, if not several thousand, have been killed, and more than 190,000 have been displaced, according to U.N. officials. As of Wednesday, about 57,500 people were taking refuge in U.N. bases.

The political struggle has spawned ethnic bloodletting between the Dinka, the tribal group to which Kiir belongs, and the Nuer, Machar’s group, raising fears of a civil war that could tear apart South Sudan.

Bor is a pivotal town for both sides because it is the heartland of the Dinka. It was the site of a 1991 massacre of Dinkas by Nuer fighters, which triggered a major split within the liberation movement that eventually secured the country’s independence from Sudan in 2011.

The rebels seized Bor last month but were pushed out by government forces. This week, however, the rebels managed to retake the town, with the help of thousands of Nuer fighters of the White Army, so named because of the white ash they apply on their bodies for protection from insects.

The bishop of Bor, Ruben Akurdit Ngong, who fled the clashes Sunday and is in Juba, told the BBC that the town was a “war zone” with “dead bodies everywhere.” “When you are in Bor town, you move around closing your nose because of the terrible smell," he said.

Meanwhile, the situation is dire for the estimated 75,000 Bor residents who fled to the town of Awerial in neighboring Lakes state. Although aid agencies distributed some food and water, sanitation issues and clean water remained a problem. Latrines were needed to prevent a public-health disaster, said François Servranckx of the nongovernmental organization Doctors Without Borders, describing the situation as “pretty delicate and precarious.”

“We ask all political leaders who have begun peace talks today in Ethiopia to urgently take steps towards cease-fire and work actively towards reconciliation,” Emma Drew, the acting South Sudan country director for the aid agency Oxfam, said in a statement Thursday.

South Sudanese military officials vow that they would never let the rebels advance further toward Juba. “They have an intention of telling their supporters that they want to come to Juba,” Aguer said. “But we are going to retake Bor. If there is fighting, it will be around Bor, not Juba.”