The incident happened when a gas cylinder being carried by a passenger exploded in the passenger train, Tezgam near the city area of Liaquatpur, a Pakistan Railways official said.
The injured have been shifted to DHQ Liaquatpur and Bahawalpur, said Ameer Taimoor Khan, DPO Rahim Yar Khan.
As per Geo news sources, 10 of the dead bodies have been identified. Whereas, identity of the other deceased persons is yet to be ascertained.
Confirming the death toll, Baqir Hussain, Head of the district rescue service said the dead bodies will be identified through DNA.
According to reports, the passengers were cooking breakfast when the blast occurred. Two adjoining coaches were engulfed in the raging fire.
The fire destroyed three of the train carriages, including two economy class carriages and one business class.
Pakistan Railways Minister Sheikh Rasheed said: "Two cooking stoves blew up. They were cooking, they had (cooking) oil which added fuel to fire."
"Most deaths occurred when people jumped off the train," he said.
He added, men from a "Tableeghi Jammat" were travelling in the carriage where the incident occurred. "Bogies were severely damaged by the fire and they have been detached from the rest of the train," he said.
Rasheed added the fire had been extinguished and cooling efforts were underway.
Prime Minister Imran Khan, while expressing his condolences, directed the authorities to ensure the provision of best medical treatment for the injured.
This is the second major train accident in Pakistan this year. In July, 24 people were killed, when the Akbar Bugti Express travelling from Lahore to Quetta collided with a stationary freight train at the Walhar Railway Station in Sadiqabad Tehsil of Punjab province.
( With inputs from IANS )