At least four foreign nationals have been killed and five security personnel injured in a bomb blast in northwest part of Pakistan on Wednesday.<br/><br/>The explosion took place near Koto Girls School in Balambut area of Lower Dir. Three of those killed in the blast are reported to be journalist.<br/><br/>The nationalities of those killed in the explosion is not clear. <br/><br/>Several students are reportedly trapped inside the damaged school building even as security forces have cordoned off the area.