At least 25 children were killed Monday in a preschool fire in south-central Niger, officials said. The children were between the ages of five and six.

Fourteen more were injured while five are in critical condition, the Ministry of National Education said in a statement.

Monday's preschool fire destroyed three thatched-roof classrooms in the city of Maradi, the ministry said.

"For the moment we cannot state the origin of the fire," regional director of education Maman Hdi said.

Classes have been suspended and three days of mourning were declared in Maradi, he said.

In Niger, overflow classrooms are often built using wood and straw when the main brick structures are unable to accommodate all schoolchildren, BBC News noted.

“It is better to hold classes under trees than in straw huts, which have become flammable graves for pupils,” Issoufou Arzika, secretary general of the teachers union, told Agence France-Presse on Monday.

This is Niger’s second major school blaze this year. The first one was in April and killed about 20 children, destroyed eight straw classrooms and 30 permanent classrooms.

The council of ministers said on Monday that following the two incidents, preschool classes must not be held in straw-roof huts.