The blast occurred overnight in the village of Mbuba, about 120 kilometres (74 miles) west of the capital Kinshasa, Kongo Central provincial governor Guy Bandu tweeted.
Mabiala Khonde, a civil-society representative in the area who passed the scene of the accident, said the truck had toppled over and was leaking fuel.
"People were running to collect the fuel," he told AFP by telephone from Matadi. "Several minutes after we heard an explosion".
Mbuba lies on the busy RN1 highway linking Kinshasa to the ports of Matadi and Boma.
"It is high time to take bold and drastic measures to strengthen transport regulations, especially for flammable products, to end to this cycle of accidents," Bandu said.
"Highway One should not be a graveyard."
More than 50 people were killed in a fuel-tanker blast in the same village in October 2018.