The truck, in which 25 people were travelling, was heading towards Lucknow when it broke the railing of Sanjay Bridge and fell into the river, SP Anand Kulkarni said.
The accident happened on Barabanki-Bahraich road, he said.
Three persons, including the truck driver, swam to safety while the others were believed to have drowned, the SP said.
Efforts were on to locate the bodies or any possible survivor, he said.