Rescue operations were under way yesterday as the number of people stranded by floods from torrential monsoon rains and glacial snowmelt climbed to 12 million across north India, officials said.
Bihar, where flood waters have affected almost 7 million people, and Assam were among the worst affected states.
"A total of 6.89 million people are reeling under the effects of the floods," said Satish Chandra Jha, the top officials from Bihar's relief and rehabilitation department.
More than 3,000 villages were waterlogged, the official added.
Nearly 1,000 people have died in India since the onset of monsoon rains in June, according to a tally compiled from officials and media reports.
Several rivers had burst their banks in the north of impoverished Bihar state while in other parts dams had been forced to discharge brimming water, the official added in the state capital Patna.
Charity Oxfam, which said it was working with local aid groups, expressed concern that a lack of clean drinking water could spark widespread waterborne illness.
"Providing safe drinking water and addressing public health is essential because it saves lives," Lalchand Garg, Oxfam's manager in eastern India, said in a statement.
At least 25 people have died due to the floods in Bihar in the past week, but the figure could rise.
Officials were also grappling to provide relief in northeastern Assam, where 5.5 million people have been displaced by floods.
"Relief and rescue workers are working on a war footing. But in this kind of a devastating flood, we know we cannot satisfy each and everybody," Assam chief minister Tarun Gogoi said.
Baby food and water purifying tablets were being dispatched, Gogoi said, but people stranded in Senimari village, just 75km east of Assam's main city, Guwahati, said they had yet to see either.
Flooding across north and northeast India claimed more than 150 lives in July.