Clashes between rival tribes in a South Sudan town have left hundreds dead, a UN official has said.

Government troops opened fire to stop about 6,000 armed Lou Nuer tribal youth from entering the town of Pibor in Jonglei state which is home to the rival Murle people, Lise Grande, the UN humanitarian co-ordinator for South Sudan, said on Tuesday.

Tensions between the two tribes over cattle rustling have mounted in recent months.

“There were more than a hundred children who were orphaned and unaccompanied,” Grande told Al Jazeera.

Our correspondent in Juba reported that hospital there overflowing with survivors of the violence.

“Juba Hospital already has limited resources,” said Al Jazeera’s Haru Mutassa.

“The arrival of more than 100 patients from Jonglei state is making things difficult.”

Unconfirmed reports based on survivor accounts given to the AFP news agency suggested up to 150 people, largely women and children, were hunted and killed after fleeing Pibor.

Grande said she saw five corpses on the southern edge of Pibor when she visited the town earlier on Tuesday.

Government forces and UN peacekeepers launched a major operation as the column of Lou Nuer fighters arrived at Pibor. The armed youths breached a perimeter of government troops in the south of the town, Grande said.

Earlier in the morning, the government claimed that its army was in “full control” of Pibor, and that Lou Nuer tribesmen were vacating it.

“Full control”

“Pibor is under the full control of the government, and the Lou Nuer have been ordered to return to their homes, and they are starting to do so,” Barnaba Marial Benjamin, the country’s information minister, said.

Thatched huts have been burned and, according to Parthesarathy Rajendran, the head of Doctors without Borders (known by its French name Medicines sans Frontieres, or MSF) in South Sudan. Thousands have been displaced, including his own staff, he said.

“Many of our staff are in the bush and we also heard an MSF clinic has been damaged and looted,” he told Al Jazeera.

“MSF is the only medical health care in the whole region, and the population is very vulnerable for all kinds of medical issues. So we are very concerned about those fleeing into the bush. They don’t have access to water, medical care or food.”

Reverend Mark Akec Cien, head of the Sudan Council of Churches, an umbrella organisation with members across the area, said they had reports of many killed and wounded in the clashes.

“The situation is very bad, there have been heavy casualties, and the Murle have fled out of the town,” Cien said from Juba. He said some armed men were reportedly chasing those running away.

“The Lou Nuer are there in the town, but others have left chasing after the Murle,” Cien said.

More than 30 people were killed in Lukangol, about 30km north of Pibor, where Lou Nuer fighters attacked last week, burning the settlement to the ground, Cien said, quoting reports from the ground.

“There are several wounded too, some say over 80, and that does not include the Lou Nuer, as they are not saying how many may be casualties from the fighting,” he said.