South Sudan President Salva Kiir has sacked the head of national oil company Nilepet, according to a government decree.
Managing Director James Thelweng Mathiang Rok was relieved of his duties, according to a decree broadcast on state television. Rok had been in the job since February.
Chol Deng Thon Abel was named as new managing director, it said.
The world's newest nation has 7 billion barrels of oil in proven reserves. But it has been mired in civil war since the end of 2013, less than two years after it gained independence from Sudan -- and autonomy over its petroleum reserves.
Oil accounts for almost 100 percent of the country's export revenue but hard currency reserves have been slashed by a fall in oil prices and in output since war broke out.
This has fuelled rampant inflation and fuel shortages are currently gripping the capital, in part due to lack of dollar reserves to buy imports and to flooding on the main road to Uganda, where fuel tankers from the Kenyan port of Mombasa make their way into the landlocked country.
The conflict has also forced nearly a third of the population of 12 million to flee the country, creating Africa's biggest refugee crisis since the 1994 Rwandan genocide.