More than 1,600 people have been killed since June in India's worst monsoon rains in 25 years, authorities said.
The monsoon, which usually comes between June and September, caused 10% more rainfall than the 50-year average and is expected to continue in October, longer than usual.
In the city of Patna, capital of Bihar state, home to about 2 million people, residents said they were struggling to walk up to the waist to buy basic necessities such as food and milk.
"The whole area is surrounded by water," Ranjiv Kumar, 65, who lives in the Ashiana district of Patna, told Reuters by telephone.
"The government is not carrying out any rescue operation and the situation is very serious here," he said.
In neighboring Uttar Pradesh, India's most populous state, heavy rains destroyed more than 800 houses and flooded vast areas of farmland.
• Heavy rains destroyed 800 houses in Uttar Pradesh.