No fewer than 20 children have died after their classrooms caught in fire in Niger's capital Niamey.

The cause of the fire in the school, located in the Pays Bas area of the city, has not yet been established.

It was gathered that the fire started around the entrance of the school, which has a population of 800 students.

The fire, which spread to 21 classrooms built with straw, trapped the children inside as the they did not have an alternative escape route.

"Without an emergency exit, many were trapped and students were forced to scale the wall to escape. Those that died were mostly children in the preschool," Reuters quoted a teachers' union official, Mounkaila Halidou, as saying.

Prime Minister Ouhoumoudou Mahamadou has visited the school to offer his condolences to the children's parents.