Fifteen people died on the spot while three others succumbed to their injuries on way to hospital, Kinnaur Deputy Commissioner DD Sharma said.
The bus carrying about 35 passengers was on its way from Sangla valley to Kalpa when the mishap took place. The injured persons are being brought to IGMC Hospital Shimla and a state helicopter has been sent to Sangla to evacuate the injured.
The driver and conductor were among the dead while all 17 injured persons have been rescued and rushed to a local hospital.
The bodies of the deceased have been retrieved and would be handed over to the family members after an "on the spot post mortem", Sharma said.
Most of the deceased are from surrounding villages of Sangla.
In another incident, two persons were buried alive when a huge mound of earth fell on their vehicle near Chulling on Chulling-Tapri diversion road.
One body has been recovered while a search is on to recover the other body as the vehicle is completely buried under huge rocks.
Chief Minister Virbhadra Singh has expressed grief over the accident and directed the district administration to provide immediate relief and adequate medical help to the injured persons.