At least 170 people are dead after a crashed oil truck exploded in the middle of a crowd who were siphoning fuel from it.
Another 50 people were injured in the blast.
The tanker had veered off the road in Maridi, Western Equatoria state.
The commissioner of Maradi county, Wilson Thomas Yanga, said that while the driver went to the nearest town for help, local residents descended on the tanker to collect some of the petrol.
He said it was not yet clear what caused the explosion.
Mr Yanga said the death toll had risen to 176, and officials feared the true number of wounded was not yet known as many may have fled the scene.
Doctors and medical staff were flying in from across the country to help with the emergency, he added.
Local hospitals have been overwhelmed, and state officials have appealed to the Red Cross and the United Nations for help.
Similar incidents have happened before in a region where fuel tankers often have to travel long distances along potholed roads and pass through poor communities.
In June 2013, at least 30 people were killed while trying to siphon fuel from a crashed tanker that later exploded.