The tanker carrying gasoline crashed and tipped into a ditch along a road in Totota, about 130 kilometers (80 miles) from the capital Monrovia on Tuesday.
President George Weah sent his "profoundest sympathy" to the families of victims.
"So far more than 40 deaths have been recorded since a tanker transporting fuel made an accident and later exploded," the presidency said in a statement.
"It is reported that some locals in the area attempted to scoop the contents of the trapped tanker when the explosion happened," it added.
"There were lots of people that got burned," said Prince B. Mulbah, deputy inspector general for the Liberia national police.
Eyewitness Aaron Massaquoi told AFP that "people climbed all on top of the truck taking the gas, while some of them had irons hitting the tanker for it to burst for them to get gas."
"People were all around the truck and the driver of the truck told them that they could take the gas that was spilling," Massaquoi said.
"He told them not to climb on top of the tanker and that they should stop hitting the tanker.... but some people were even using screwdrivers to put holes on the tank."
Liberia's chief medical officer Dr Francis Kateh said that dozens had been badly burned.
"Dozens of injured people from the incident have been transported to various hospitals," the presidential statement confirmed.
"The President (Weah) has said health authorities have got his full backing to beef up manpower and equipment where necessary in their frantic attempt to save lives."