Officials had originally put the death toll at 73 following the massive blast on Thursday in Tete province in the country's remote western region near Malawi.
By Friday, officials had counted 60 bodies in mortuaries as recovery efforts continued.
"In the accident, 108 people were injured, 96 of whom are still being kept in for treatment at Tete Provincial Hospital," government spokesman Mouzinho Saide said at a press conference in Maputo.
"The cabinet has created a commission of inquiry to investigate the circumstances, causes and responsibilities for this accident."
According to authorities' initial accounts, the truck was carrying petrol from Mozambique's port city of Beira to neighbouring landlocked Malawi.
The driver took a detour and stopped close to the Malawi border in the village of Caphiridzange to sell petrol to local people, a common practice in Mozambique.
"The truck drivers were transferring petrol into a smaller truck and they fled when they noticed there was an (electrical) short circuit," Emilia Moiane, an information ministry director, told AFP.
"Seeing the truck had been abandoned, locals came to syphon petrol off, not knowing that the truck was already burning inside." One of the truck drivers was from Mozambique and the other from Malawi, officials said.
President Filipe Nyusi told reporters that "tragedy has knocked on our door" with the high loss of life.
"What is important now is to take action and help the affected," he said.
Photographs and video footage from the hospital in Tete showed badly burned children arriving for emergency care and adults lying on hospital beds.
"We still have a lot of cases in a critical condition, including children and two pregnant women, out of 38 cases in total," Tete hospital director Veronica de Deus said.
"The vast majority of patients have severe burns. Some have 80 to 90 percent of their bodies burnt," she said on public broadcaster TVM.
Authorities said many of the dead would be buried in a mass grave, and announced that three days of national mourning would start on Saturday.
A plastic surgeon and other emergency health staff have been sent from Maputo, 1,500 km (930 miles) by road, to help deal with the large numbers of injured, including 17 children.
The government in Mozambique, one of the world's poorest countries, recently increased the price of fuel after the value of the local currency -- the metical -- fell sharply.
The metical has collapsed by 70 percent against the US dollar this year after falling 36 percent in 2015.
The Tete provincial government has appealed for emergency food aid and transport assistance for affected families.