The accident took place last Wednesday on a road some 250 kilometres (155 miles) west of the capital Juba, close to the small town of Maridi, with the victims including locals who tried to scoop up the fuel spill.
"The death toll has reached about 203," Health Minister Riek Gai Kok said in a statement read on state radio, updating an earlier toll of 186.
Over 150 people wounded in the blast are still in hospital, with 65 of some of the worst burned having been taken to the main hospital in the capital Juba.
The country declared three days of national mourning after the blast.
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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.
One of the worst such accidents was in Nigeria in 1998, when over 1,000 people died in the southeastern Delta State when a pipeline exploded as people tried to steal fuel.
South Sudan is 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.