Four paramilitary troopers and three policemen were injured on Monday in an explosion in Pulwama district of Jammu and Kashmir, police said.The explosion occurred in Pulwama town inside a vehicle in which four troopers of the Central Reserve Police Force (CRPF) and three local policemen were travelling, the police said.While initial reports said the explosion had been caused by a grenade hurled at the vehicle by separatist guerrillas, top police officials now say that a tear smoke shell exploded accidentally inside the vehicle.The injured have been taken to hospital for treatment.Meanwhile, a gunfight started on Monday between a group of heavily armed guerrillas and the security forces in the Rajwar forests of Kupwara district of the state.