Khartoum, Sudan (CNN) -- South Sudan's government has gained control of a remote town that had been under attack by fighters from a rival tribe, the nation's information minister said Monday.
Some of the thousands who fled into the bush have begun to return to Pibor, said Barnaba Benjamin, South Sudan's minister of information and broadcasting.
Earlier Monday, a military official said that roughly 4,000 army and police reinforcements were on the way to Pibor after weekend attacks.
The United Nations also said it sent a battalion of peacekeepers to Pibor last week amid reports that members of the Lou Nuer tribe were marching toward the town, home to the Murle tribe, after attacking the village of Lukangol.
Ethnic tensions in the South Sudan state of Jonglei have flared as tribes fight over grazing lands and water rights, disagreements that have dissolved into cattle raids and abduction of women and children.
Fighters first struck Pibor on Saturday, attacking a portion that was out of the reach of U.N. peacekeepers, said Col. Philip Auger of the South Sudan army.
Benjamin said there were no casualties and no direct confrontation between the two tribes there, because most of the Murle had fled when they heard the Lou Nuer were approaching.
The nonprofit group Medecins sans Frontieres, also known as Doctors Without Borders, was still trying Monday to reach 117 of its 130 aid workers who fled into the bush along with the populations of Pibor and Lukangol before the attack, said Sarathy Rajendran, head of the group's South Sudan mission.
Rajendran said the organization was afraid for its workers' safety and "very concerned" for the people of Pibor and Lukangol.
"We believe tens of thousands are currently displaced without access to water, food and health care," he said. "The situation is tense. We don't know what is happening on the ground, so we are monitoring the situation."
The nonprofit's clinic in Lukangol was "burned and looted," spokeswoman Emily Linendoll said, adding that its Pibor clinic "has been targeted."
The attack on Pibor follows reports last week that Lou Nuer fighters raided Lukangol, burning it to the ground and forcing thousands to flee toward Pibor.
At least 50,000 people have fled the violence in the state that began last year, said Quade Hermann, chief of radio at Radio Miraya, a U.N.-backed radio station in South Sudan.
She similarly said that the town of Pibor is secure now, though the situation there remains fluid. The people who fled remain scattered, and the United Nations is working on a plan for how to distribute humanitarian aid, Hermann said.
U.N. Secretary-General Ban Ki-moon last week urged the groups to work with the government of South Sudan to find solutions.
South Sudan's vice president, Riek Machar, is leading an initiative to bridge the differences between the Lou Nuer and Murle tribes, including encouraging the armed groups to disband and go home, the United Nations has said.
The violence in Jonglei state is the latest to rock South Sudan, which officially gained its statehood in July after separating from the north.