South Sudanese military officials say the town of Pibor came under siege by as many as 6,000 attackers who torched a hospital. South Sudan Col. Philip Auger told CNN Saturday\'s attack overwhelmed 800 soldiers and United Nations peacekeepers were too far away to assist. There was no report on the number of casualties by midday Sunday. The remote town is home to members of the Murle tribe, which for decades has been in conflict with the Lou Nuer tribe over water rights and cattle rustling, the report said. Last week, reports emerged the Lou Nuer tribe had burned the Murle town of Kukangol to the ground, CNN said. Meanwhile, officials with the Doctors Without Borders medical charity told the BBC there were concerns for staff in the Pibor attack. Early Sunday, spokesman Parthesarathy Rajendran told the broadcaster of its 130 medical staff in the town, only 13 had been located. He said there were hopes the rest had fled into the surrounding bush land. South Sudan has been wracked with internal tribal violence since seceding from Sudan in July 2011.