NEW DELHI: Fresh gunshots were heard while an Improvised Explosive Device exploded on Sunday during the combing operation at the Indian Air Force frontier base in Punjab's Pathankot, which was attacked by Pakistani terrorists on Saturday.Three security personnel belonging to the National Security Guard were injured in the blast, news agency PTI said. The combing operation that began late on Saturday evening is still on.There is speculation that a sixth terrorist could still be holed up inside the air base. The attackers, five of whom were killed on Saturday, are believed to belong to Jaish-e-Mohammad headed by Maulana Masood Azhar of the Kandahar hijack episode.The attack came a week after Prime Minister Narendra Modi made an unscheduled visit to Pakistani counterpart Nawaz Sharif in an effort to revive talks between the two countries.Officials said the attack on the military base, just 25 km from the border with Pakistan, bore the hallmarks of previous assaults by Pakistan-based militant groups, underscoring the fragility of recent efforts to revive bilateral talks between the uneasy neighbours.Meanwhile, in the seventh casualty among the security forces, Lt Col Niranjan Kumar of the National Security Guard died in hospital on Sunday morning. He was injured in an IED blast while retrieving the body of a terrorist.An eight-member team of the National Investigation Agency (NIA) has reached Pathankot in Punjab to take over the probe into the attack.(With PTI inputs)