At least 32 children were burnt to death and 18 others were injured when a bus carrying them caught fire Sunday in north Colombia's Magdalena province, authorities said.

It is the deadliest road accident involving children in Colombia's history.

Firefighters recovered 32 bodies believed to be children from the bus but horrific burns made their identification difficult, Magdalena police chief Adan Leon told local media.

The 18 injured children have been sent to three hospitals nearby.

The bus departed Sunday noon from a church in the city of Fundacion, about 750 km north of the capital of Bogota, to take the children home from church events.

Witnesses said the bus had traveled about four blocks when it suddenly caught fire and exploded. Some older children managed to climb from the windows but the younger ones were trapped and burned to death.

The police said investigations into the cause of the tragedy was underway.

It was speculated the accident was related to fuel smuggling or misconduct by the driver.

A lot of Colombians smuggle cheap gasoline from Venezuela and transport it hidden in private cars and buses.

Major Eduardo Velez, coordinator of Magdalena province's emergency response corps, said there was a canister of gasoline inside the vehicle and the fire spread very fast.

He said the fire started after the driver attempted to start the malfunctioning bus by pouring fuel into the engine, which he accessed through the floor of the cabin. The driver escaped unharmed and was being questioned by police, he said.

Colombian President Juan Manuel Santos went to Fundacion to review rescue efforts and console the families of the victims.

"The entire country is in mourning for the death of these children," Santos said upon arriving in Fundacion, accompanied by his wife.