Police official Bikash Shrestha says the exact number of missing is unknown. He says the boat was carrying at least 35 people and only a dozen have been accounted for.
The boat capsized late Tuesday on the Kamala river. The river is located 250 miles (400 kilometers) southeast of the Katmandu. By the time rescuers reached the site it was already dark.
Another police official Hari Bahadur Pal says the boat was very old, overcrowded and the river was flooded by the monsoon rains.
The search continued Wednesday morning.