"You could be prepared for a normal flood but this is not a normal flood, it's a complete disaster," he told Al Jazeera.
Mishra said the Indian army and the national disaster response force were working to evacuate one million people from their homes.
"We are keeping them in relief camps and we think that for the coming several months, they have to be kept in these camps."
Indian emergency services were rushing supplies to the flood victims as food riots erupted in eastern India, where about 250,000 houses have been destroyed.
Akhilesh Prasad Singh, a junior agriculture minister, said one million tonnes of rice and wheat would be doled out.
One person was killed in Madhepura district when angry villagers fought among themselves over limited supplies of food and medicines at overcrowded relief centres.
|Drinking water is loaded on to a train for flood-affected residents
The Kosi river in Bihar, one of India's poorest states, smashed through mud embankments and changed course last week, unleashing huge walls of water that inundated hundreds of villages and towns.
Some 10,000 of Madhepura's 45,000 residents fled soon after Kosi began shifting from its course on August 18.
Torrential rains have killed more than 1,000 people in South Asia since the monsoon began in June, mainly in India's northern state of Uttar Pradesh, where 725 people have lost their lives.
Other deaths were reported from Nepal and Bangladesh.
"A couple of days ago just a handful of districts were affected and today there are 15 which are under water," Mukesh Kumar told an AFP news agency correspondent in Madhepura, some 225km from the state capital Patna.
"We need a massive rescue effort," said local council member Kumar.
Stranded villagers waved at passing helicopters and sent text messages to local authorities from rooftops of flooded buildings.
"Time is running out for me and there is no relief in sight and I have not eaten for days," a message from flood victim Sanjeev Kumar read.
Among those hit was Shanti Devi, who dragged her two children out of their house as the waters rose.
"The government is asking us to escape but we have nowhere to go," the 32-year-old mother said.
Nitish Kumar, the Bihar chief minister, advised panic-stricken people to "run" in an overnight address on state-run radio.
"Time is running out for these luckless people," Narendra Narayan Yadav, the state revenue minister in Madehpura where the swirling water has overrun train tracks and the main highway, warned.
People were using rowboats or a few backroads still above the rising water levels to flee.