The search operation has begun at the Islamabad airport after some unknown person on phone threatened to blow up Islamabad airport at 12:40pm Thursday. According to sources some unknown person made a phone call at PIA head office in Karachi saying Islamabad airport would be blown up in a suicide attack at 12:40pm today. Meanwhile, security has been heightened at the airport after the suspected phone call. The passengers have been immediately taken to some safe place after PIA flight arrived here from Birmingham.The sources say there is tense situation at the airport while security forces are undergoing search operation.