The ill-fated boat capsized in Saryu river at Bahraich, 135 km from here. It was carrying villagers who had gone to attend a fair on the occasion of Kartik Poornima.
There were 25 people on the boat, out of which 13 were rescued. There is, however, no trace of the remaining dozen, including five children.
A district official told IANS that the rescue and search operations were not carried out at night. He said by day break, divers would be sent into the river to locate the missing people.
The incident took place near the Avseripurva village when pilgrims were returning from a fair at the Takia Ghat. The missing people belong to neighbouring villages, Ramsahaypurva, Narainapur, Chandanpur, Sankalpa and Gokulpur, police said.