A hand grenade exploded Tuesday, wounding two school children, one critically, as they were playing with it in Upper Dir school, police said.

The incident took place a primary school in Dhob Darra of Upper Dir valley, where Taliban militants have been active.

Two children who came to school early found a grenade lying on the ground. They picked it up and started playing with it before removing its pin, senior police official Zahid Khan told a foreign media agency.

One of the children was critically wounded while the other had light injuries, Khan said, adding that police suspect militants had left the grenade there.

Taliban militants stormed a school in Peshawar in December 2014, killing more than 150 people, mostly children, in Pakistan’s deadliest-ever terror attack.

Militants opposed to co-education have destroyed hundreds of schools, mostly for girls, in north-west Pakistan, while the shooting of Malala Yousafzai in Pakistan’s Swat Valley in 2012 shocked the world.

The army launched an operation in June 2014 in a bid to wipe out militant bases in the tribal areas and so bring an end to the bloody insurgency that has cost thousands of civilian lives since 2004.