A helicopter carrying 11 foreigners and six Pakistanis crashed on a school building in the country's mountainous north on Friday, the army and police officials said, resulting in three critical injuries.

"According to the first information we received, three people have been injured, their condition is critical," a police official in Nomal village of the Naltar valley told AFP.

A Pakistani journalist who was travelling in a separate helicopter said she saw the helicopter crash into a school and it was on fire.