Khartoum, Sudan (CNN) -- Thousands of fighters attacked a remote town in South Sudan despite the presence of United Nations peacekeepers in the area, a military official said Sunday.
Authorities were counting casualties from Saturday's attack by 6,000 fighters that occurred in a portion of the town of Pibor that was beyond the reach of the peacekeepers, said Col. Philip Auger of the South Sudan army.
Most staffers affiliated with the non-profit group Doctors Without Borders, also known as Medecins Sans Frontieres, who were in the area "fled into the bush, along with the populations" of Pibor and the village of Lekongle before the attack, said Emily Linendoll, a spokeswoman for the group.
The non-profit's clinic in Lekongle was "burned and looted," she said, adding that its Pibor clinic "has been targeted."
In addition to the threat of violence, Linendoll said that Doctors Without Borders believes "thousands are currently without access to water, food or health care" in the region.
"(Doctors Without Borders) is extremely worried for the populations of ... Lekongle and Pibor," she said.
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.
The United Nations said it sent a battalion of peacekeepers to Pibor on Friday amid reports that fighters -- members of the Lou Nuer tribe -- were marching toward Pibor, home to the Murle tribe.
The attack on Pibor follows reports last week that Lou Nuer fighters raided the town of Lukangol, burning it to the ground and forcing thousands to flee toward Pibor.
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 U.N. 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.
Fighting erupted between Sudan's army and South Sudan rebels in Southern Kordofan even before independence was formalized. The violence has since spread to other areas.
Within a few months, refugee camps filled as fighting in the border states of Southern Kordofan and Blue Nile intensified, displacing an estimated 400,000 people, according to the United Nations and aid agencies, such as Medicins Sans Frontieres (MSF) or Doctors Without Borders.
That, in addition to the crisis in the Darfur region -- where war broke out in 2003 -- qualifies Sudan as one of the world's largest humanitarian crises, according to the United Nations.
CNN's Claudia Dominguez contributed to this report.