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State media reported most of the victims were burned beyond recognition.
Six people were taken to St. Peter's Hospital, where another three died.
Many the victims were burned beyond recognition, Spot FM radio said.
"Charred bodies were scattered on the road while others are still trapped under the wreckage of the haulage truck," Agence France-Presse quoted the radio announcer as saying.
The truck was carrying mourners to a funeral when it allegedly side-swiped the tanker in the sugar plantation district of Chisumbanje in the country’s southeast.
"We are investigating. So far, we know that more than 20 people died," a police spokeswoman told Reuters.