Twelve children have been killed in north-western Pakistan after playing with a bomb they mistook for a toy, local officials say.

The children died after the bomb, which resembled a football, exploded on Saturday in Lower Dir district of North West Frontier Province.

The children - seven boys and five girls - were between five and 13 years old, officials say.

They found the device outside a local school as they were going home.

Officials say an investigation is under way to determine how the children got hold of the bomb.

Low Dir is part of Malakand division in North West Frontier Province.

Taleban militants there have recently agreed a peace deal with the government.