Four persons, including a policeman, were injured when a powerful blast rocked Pattan, near Srinagar, on the Muzaffarabad route minutes after the bus passed the town on Thursday.
The bomb was planted in a shop at Pattan Bazar. The blast occurred at around 12.25 pm.
Several firing shots were heard after the blast.
Sources said that it was believed that the firing was resorted to by security personnel dotting the highway to foil plans by militants to target the vehicle.
Four outfits, which had threatened the passengers against the travel on the Srinagar-Muzaffarabad bus claimed that its militants targeted the vehicle at Singhpora Pattan.
A caller introducing himself as Samir Abdullah, spokesman of the four outfits - Al-Nasireen, Al-Arifeen, Save Kashmir Movement and Farzandan-e-Millat - telephoned news organisations saying their members hurled a grenade and followed it by firing on the bus.