Hundreds of villages have been cut off, with many people climbing to higher ground while others are stranded, with diminishing food and water supplies.
Dasgupta said: "Telephone lines were snapped and electric poles uprooted.
"Nearly 2.5 million people were forced from their homes by flood in over 300 villages during the past four days."
Helicopters have been dropping aid to those people ground aid teams have been unable to reach.
R K Das, a defence spokesman, said that thousands of people were taking refuge on rooftops.
Nearly a million people in the neighbouring state of Orissa have also been affected.
In Assam state in northeast India, more than 350,000 people have moved to government camps for safety.
Monsoon rains are typical from June to September in India.