The Press Trust of India (PTI) news agency quoted Defence Minister George Fernandes as describing the flood situation as "a human tragedy beyond imagination."
"It looks like a deluge," PTI quoted Fernandes as saying after he made an aerial survey of the flood affected regions.
The floods have so far claimed at least 1,586 lives in the three states of West Bengal, Assam and Uttar Pradesh, officials say.
In eastern West Bengal state, the number of dead over the past two weeks rose to 136, and the number of those affected increased to 4.5 million.
The state has already spent 400 million rupees ($9.4 million) on relief work.
"Relief efforts are being made but there is a major transportation problem, the main bottleneck is railways," said I.S. Ingty, a state relief official.
Officials said they were cautiously watching the weather forecast which predicted heavy rains in the next 48 hours.
The Brahmaputra river and its tributaries have submerged more than 800,000 hectares of land in the oil-and tea-rich state of Assam, disrupting the telecommunication network and washing away highways.
The river, which flows into Bangladesh before emptying into the Bay of Bengal, has caused huge damage there, killing 750 people and swamping 70 percent of the country.
The state government of India's northeastern Assam deployed hundreds of doctors to prevent outbreaks of contagious disease among the 3.6 million people who have been evacuated from homes, officials said.
The floods have claimed about 200 lives in the state over the past four months.
In northern Uttar Pradesh state, officials said they were waiting for river waters to recede before they could use newly-installed pumps to rid some of its worst-flooded cities of water.
Floods and landslides have so far claimed 1,250 lives and waterlogged 49 of the state's 83 districts.
Senior state official Naresh Dayal said drinking water in tin cans was being distributed along with chlorine tablets in the largest relief operation the state has ever undertaken, to try to prevent epidemics.
"At least it will reduce the level of contamination," Dayal said.
- Reuters - Thomson Reuters Foundation
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