"They were killed when the oil tanker exploded, there were at least 85 people dead," Ateny Wek Ateny told AFP, citing local officials. Over 100 others suffered burns, local government official John Skia told South Sudanese Eye Radio.
Those wounded were taken to Maridi hospital, with a doctor there, Chandi Savior, telling Radio Tamazuj that they were struggling to help all those with burns, with supplies of basics such as oxygen and simple pain killers low. Medics are "not really able to calm down this pain", Savior told the radio.
Fuel leaks and oil tanker accidents in Africa often draw huge crowds scrambling to scoop up the fuel, resulting in many deaths due to accidental fires.
But South Sudan is also in the grip of a dire economic crisis sparked by over 21-months of civil war, which has caused rampant inflation and soaring prices of basics, including food and fuel.
The violence has left tens of thousands of people dead and the impoverished country split along ethnic lines.
Over two million people have fled their homes in a war marked by gang rapes and the use of child soldiers.
The government and rebels signed a peace deal on August 29, but the ceasefire - the eighth agreed - has been repeatedly broken as fighting continues.