The mishap took place this evening as a boat carrying about 40 people lost control and capsized in the river near Narayanpur ghat in Patkholi police station area, Sub-Divisional Officer (SDO) Manzoor Alam said.
"While about 25 people swam to safety, at least 15 others are still missing," he said.
Divers have been pressed into service to rescue the missing people, Alam said, adding that senior officials were camping on the spot to supervise rescue works.
The occupants of the ill-fated boat were farmers returning to their home in Bagaha town from Gandak Diara after finishing farm works, the SDO said.