The Himachal Road Transport Corporation bus, carrying about 40 persons, was on its way from Palampur to Ashapuri temple when it fell down from a hilltop on Palampur-Ashapuri road, 40-km from here, officials said.
The number of casualties was likely to be heavy, they said.
Besides darkness, the soil has become slippery due to heavy rains in the area, hampering the rescue operation as trudging on steep and slippery slopes was risky.
So far only one dead body has been recovered but the chances of survival of passengers were very remote, officials said.
The officers of district administration, including the SDM, DSP, Regional Manager transport, police, fire brigade and medical have rushed to the spot but are yet to reach the bus, officials said.