(CNN) -- South Sudanese government forces have recaptured the key town of Bor from rebels after fierce fighting, the town's mayor said Saturday.
Bor, the strategically important capital city of Jonglei state, north of South Sudan's capital, Juba, has changed hands several times in a month of fighting.
The mayor of Bor, Nhial Majak Nhial, told CNN on Saturday that the rebels were fleeing and that government forces were on the ground.
He said that the defeated rebel force numbered well over 15,000, and that the rebels had lost a lot of people in the fighting.
The conflict has also taken a heavy toll on civilians in the town. Local agencies estimate that between 1,200 and 1,400 civilians were killed when rebels held Bor between December 17 and 24, the mayor said.
It's not yet known how many more civilians lost their lives between December 31, when the rebels recaptured the town, and the government's reported victory Saturday.
But Nhial said more atrocities had been committed while the town was again in rebel hands.
"They have taken all the cows, they have killed all the people, all the people who they've found, they've killed them," he said.
"There will definitely be more than 1,200 civilians killed," he said. "Absolutely more than that. The local population is more than 225,000, and they cannot all run. Anybody who couldn't leave the town or the area is killed by the rebel forces. They've even killed my own aunt."
The mayor said there was no food in his town, much of which has been burned to the ground, and he made an urgent appeal for humanitarian aid.
The area around Juba, about 125 miles (200 kilometers) away, is now safe, he added.
The country erupted into violence on December 15, when rebels loyal to ousted Vice President Riek Machar tried to stage a coup. Since then, militia members loyal to Machar have battled government forces loyal to President Salva Kiir.
The conflict has taken on an increasingly ethnic dimension, with widespread atrocities reported.
Representatives of the government and rebels have been holding talks in Addis Ababa, Ethiopia, but violence continues to wrack the world's newest country.
U.N. Assistant Secretary-General for Human Rights Ivan Simonovic warned Friday, at the end of a four-day visit to South Sudan, that one month of fighting had "set South Sudan back a decade."
"Mass atrocities have been committed by both sides. During my visit, I have received reports of mass killings, extrajudicial killings, arbitrary detention, enforced disappearances, sexual violence, the widespread destruction of property and the use of children in the conflict," Simonovic said.
"Thousands of people have been killed and hundreds of thousands are now displaced, with about 70,000 people seeking protection in U.N. camps and 30,000 in the two U.N. compounds in Juba alone," he added.
Medecins Sans Frontieres said Wednesday that heavy fighting over the preceding few days had wounded hundreds of people and displaced thousands.
The humanitarian group, also known as Doctors Without Borders, said its teams had treated 116 people suffering from gunshot wounds in the towns of Malakal and Nasir in Upper Nile state amid clashes there and elsewhere.
The charity said Friday that it had been forced to suspend its activities in Malakal after its compound was looted by armed men who threatened its staff. "This leaves thousands of people without much needed surgical and general health care -- a matter of huge concern to us," general director Arjan Hehenkamp said.
Museveni: Ugandan forces involved
The conflict has forced many South Sudanese to flee their homes.
The United Nations said this week that about 413,000 people are displaced within South Sudan in total, many of them women and children.
Another 80,000 people have fled to neighboring countries, more than half of them to Uganda, the United Nations said.
Speaking Wednesday at a summit of the International Conference of the Great Lakes Region in Angola, Ugandan President Yoweri Museveni said some of his country's troops were fighting alongside South Sudanese government forces, the Sudan People's Liberation Army.
On January 13, he said, "the SPLA and elements of our army (the Uganda People's Defence Forces) had a big battle with these rebel troops at a point about 90 kilometers from Juba where we inflicted a big defeat on them."
He added, "Unfortunately, many lives were lost on the side of the rebels. We also took casualties and also had some dead."
Museveni criticized South Sudan's governing SPLM party for failing to prevent what he said began as an internal political struggle from turning into a deadly conflict.
"To turn a political problem into a military one, having mismanaged the political problem itself in the first place is not acceptable," he said.
South Sudan seceded from Sudan in 2011 after decades of war.
Journalist Samson Ntale and CNN's Arwa Damon contributed to this report.