Public television, citing witnesses, said the fire had broken out at around 1500 GMT, 'when the victims were in class'
Some 20 children died on Tuesday after several classrooms caught fire at a school in Niger's capital Niamey, the fire service said.
"Twenty-one classrooms in straw huts caught fire. Around 20 children were trapped in the blaze," fire service commander Sidi Mohamed said on public television.
"Rescuers set off rapidly and the fire was put out... but the power of the fire was enormous," he added. "The children were not able to escape."
Public television, citing witnesses, said the fire had broken out at around 1500 GMT, "when the victims were in class."
The cause of the fire was not yet known.
Prime Minister Ouhoumoudou Mahamadou travelled to the scene of the tragedy, on the edge of the capital, to offer his condolences to the children's parents.
Niger ranks as the poorest country in the world under the UN's 189-nation Human Development Index.
Authorities in the Sahel country have built thousands of straw huts to serve as classrooms, with children sometimes seated on the floor.
Fires in these buildings are fairly common due to the flammable materials used, but such accidents have rarely proved fatal.