Air force helicopters and troops were trying to get food to people in the stricken areas of Bihar state that were inundated by flood waters last week after torrential rains caused the Kosi river in neighboring Nepal to burst its banks.
The Bihar state government issued a plea to relief agencies to step in and help get food and shelter to the residents.
"It is not a normal flood, but a catastrophe," said Bihar Chief Minister Nitish Kumar after making an aerial survey of the ravaged districts.
Kumar said more than 1 million people were cut off from the rest of the country because the floods had washed away roads and made railway lines impassable.
India's monsoon season, which lasts from June to September, brings rain vital for the country's farmers but also massive destruction. Floods, mudslides, collapsing houses and lightning strikes kill hundreds of people every year.
This year's monsoon has killed more than 330 people in India so far. In 2007, monsoon floods killed more than 2,200 people across South Asia and left 31 million others homeless, short of food or with other problems. The United Nations called last year's floods the worst in living memory.
Officials said they were struggling to cope because the Kosi river changed its course after bursting its banks, flooding areas not usually affected by the seasonal monsoon.
"We are in for a long battle as the floods have come in areas which are not prone to it," said the state's Disaster Management Secretary Prataya Amrit. He said the areas lacked facilities and relief centers for dealing with floods.
Kumar said he had appealed to the Indian Embassy in Nepal to secure permission for India to send in workers and equipment to try to mend the breach in the river.