PESHAWAR, PAKISTAN (AFP) - Three children were killed after a mortar shell they mistook for a toy exploded in a field in Pakistan's northwestern Swat valley, police said on Thursday (Nov 22).

The district was under the de facto control of the Pakistani Taleban in 2007-2009 until a military operation ended their rule, and has since seen sporadic militant attacks.

"The shell exploded when the children were playing with it and some started hitting it with stones," local police official Sayed Wali Khan told AFP.

Officer Khan said one child was immediately killed in the explosion - which occurred in Nela Gram village on Wednesday - while two others died in a local hospital.

The dead included two brothers and their cousin, aged between four and nine years' old. Three others sustained injuries and were receiving medical treatment.

The Swat valley was home to Nobel Peace laureate Malala Yousafzai.

When it was held by the Pakistani Taleban, they imposed a harsh brand of Islamic law and carried out public floggings and executions.