More than 40 people have been killed and dozens badly burned when a petrol tanker exploded in central Liberia, the country's chief medical officer said Thursday. 

Witnesses said people clambered onto the lorry to try to collect petrol leaking from its tank after it crashed and tipped into a ditch along a road in Totota, about 130 kilometres from the capital Monrovia on Tuesday.

Local Aaron Massaquoi told AFP that "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 in the tank".

The grisly task of establishing the death toll has been made more difficult because the victims were so badly burned, said Liberia's chief medical officer Francis Kateh.

He told AFP that his teams were "going door-to-door to establish the number of missing people".   

Children burned

Liberia is the tenth poorest country in the world, with more than half its five million people living in often dire poverty.

President George Weah, the former AC Milan striker who is due to step down as head of state on January 22, sent his "profoundest sympathy" to the families of victims.

"So far more than 40 deaths have been recorded," 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.

Police initially thought around 15 people had died with another 30 hurt, but the death toll has shot up since. "There were lots of people that got burned," said Prince B. Mulbah, deputy inspector general for the Liberia national police.

Another police officer, Malvin Sackor, said some locals were collecting the leaking petrol when the tanker exploded.

Chief medical officer Kateh said dozens had been badly burned.

Among the "patients most badly hurt are children and a pregnant woman", local health chief Cynthia Blapooh told the newspaper Front Page Africa.

"Dozens of injured people from the incident have been transported to various hospitals," the presidential statement confirmed.

"The President has said health authorities have got his full backing to beef up manpower and equipment where necessary in their frantic attempt to save lives."