This handout photograph released by Rescue 1122 shows a bullet-ridden school van at the shooting incident site in Attock. AFP

The children were on their way to school when the attack happened around 8:00am in a sparsely populated part of Attock city in the country's Punjab province.

"Two children were killed and five children were injured when gunmen indiscriminately opened fire on a school van," local police spokesman Waseem Babar told AFP.

The gunmen, who fled the scene, were embroiled in court cases with the family of the driver, who was also hurt in the shooting, according to Babar.

The girls killed were aged 10 or 11, he said.

Prime Minister Shahbaz Sharif said in a statement that such an attack on "innocent children is (a) very cruel and brutal act."

Agence France-Presse