"The toll could rise as the rescue work is still on," Superintendent of Police Ashok Kumar told IANS over phone from the spot.
The exact number of passengers travelling in the bus was not yet clear.
Divers of the Bhakra Beas Management Authority (BBMB) have been summoned for the rescue operation in the reservoir, he added.
The accident spot is some 90 km from the state capital.
The bus was on its way from Rishikesh to Bilaspur town, both located in Bilaspur district, when it met with an accident near Rayian village.
Witnesses said rescue workers had a tough time retrieving the bus from the reservoir.
Most of the victims were locals.