At least six children have been killed and 25 others injured in an explosion at a primary school in north-west Tanzania, witnesses say.

Reports say the pupils were playing with a grenade they had found.

Three were pronounced dead at the time they arrived at hospital. The others succumbed to their wounds, Rulenge hospital's doctor in charge, Sister Maria Goreth Fredricks, told the BBC.

The school is in Kagera region near the border with Burundi.

The BBC's Leonard Mubali in Tanzania reports that the region hosts refugees from neighbouring Burundi, some of them ex-soldiers, who could have abandoned their weapons in the area.