Six children were killed, and another 15 injured, after picking up a hand grenade while walking to school in Tanzania. (File picture)

Six children were killed after picking up a hand grenade they found while walking to school in western Tanzania on Wednesday, officials said.

"As they were heading to school this morning, one of the pupils picked up the bomb mistaking it for scrap metal. The pupil was planning to sell it to get money to buy exercise books," said regional police commander Agustino Olomi.

The accident happened in the country's Kagera Region, near the Burundi border. Fifteen other children were injured in the blast.

Goleth Fredrick, chief medical officer at the local Rulenge hospital, said three of the children were killed immediately, while three died later from their injuries at the hospital.

Some 15 other pupils were injured by the explosion, he said. It was unclear where the grenade had come from.