Thirty-one children are dead after fire engulfed their bus, triggering an explosion in northern Colombia.

Another 25 people including an adult were seriously burned in the blaze, in the town of Fundacion, in the department of Magdalena, Red Cross spokesman Cesar Uruena said on Monday.

“The injured have second and third-degree burns, and many are still in a critical condition” in hospitals in the area and in the city of Santa Marta, Uruena said.

The children were between the ages of one and eight and returning home after a religious service, Fundacion Mayor Luz Stella Duran told reporters.

Local police said the bus exploded around noon and the fire was so intense they had problems determining the number of casualties.

Stella Duran said according to survivors the bus driver had been handling a container with petrol aboard the vehicle. Police initially blamed the fire on a mechanical problem.

The driver has vanished, and angry locals went to his home and threw rocks at his windows, the mayor said.

Colombian President Juan Manuel Santos travelled to Fundacion after a final re-election campaign rally on Sunday in Bogota ahead of May 25 presidential elections.

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

Santos said the government would pay for all hospital and funeral expenses relating to the accident.

The bus, which was being used by an evangelical church group, caught fire one block away from the church where the children had earlier attended services.

The Red Cross said it was sending four psychologists and two volunteer specialists in psychosocial aid to help the wounded and the relatives of those killed.