The private boat sank near Mirganj in Murliganj block of the district this evening, superintendent of police O N Bhaskar said.
The number of people on the boat, the dead and survivors was not immediately known.
The rescue teams of the Army and National Disaster Response Force have been pressed into service to locate the bodies and possible survivors, he said. The operation has been hampered due to darkness and strong currents in the river.
More from rediff