Zimbabwe Broadcasting Corporation published photographs showing the destroyed vehicle on a highway between the South African border and the second city Bulawayo.
The bus driver, Ndabazinhle Sikhosana, who survived with minor burns said the "fire started after an explosion".
Local media reported that some passengers escaped through windows.
At least 42 passengers were killed, Zimbabwe police said Friday, after a suspected gas cylinder exploded on a bus, with pictures showing the burnt-out wreckage of the vehicle.
The Zimbabwe Broadcasting Corporation published photographs showing the destroyed vehicle on a highway between the South African border and the second city Bulawayo.
"At the moment we know that more than 42 people died," police spokeswoman Charity Charamba told AFP after the accident late on Thursday, with state media reporting that a cooking gas cylinder belonging to a passenger was believed to have exploded igniting a fire.
"Our police officers are at the scene," Charamba added.
The state-owned Herald newspaper said on its Twitter feed that "it is suspected a gas tank belonging to one of the passengers caused the inferno in the bus.
"Dozens have been confirmed dead and several others injured through burns."
The paper cited the bus driver, Ndabazinhle Sikhosana, who survived with minor burns saying the "fire started after an explosion".
ZBC reported that injured survivors being treated at Gwanda town hospital alleged that a gas smell engulfed the bus after dark before the conductor switched on the lights and the bus caught fire.
It said about 30 people were injured, adding the vehicle was travelling from Zvishavane to the Beitbridge crossing via the town of Filabusi when the explosion happened.
Philisani Ndebele, a police officer who was at the scene of the disaster, told the regional Chronicle daily newspaper that some bodies were burnt beyond recognition.
Local media reported that some passengers escaped through windows.
Last week, 50 people were killed when two buses collided on a road between the capital Harare and the eastern town of Rusape.
President Emmerson Mnangagwa said road safety had to be improved after that crash.
Traffic accidents are common in Zimbabwe, where roads are riddled with potholes due to years of underfunding and neglect, and driving standards can be poor.
In June last year, 43 people were killed in a bus crash in the north, along the highway leading to neighbouring Zambia.
