Millions of people were forced to flee their homes in northeast India as the death toll rose from raging floods triggered by heavy monsoon rains, a minister said.

'The situation has turned devastating overnight, drowning five more people in separate incidents and displacing another three million in 15 districts,' Bhumidhar Barman, relief and rehabilitation minister of Assam state, told Agence France-Presse.

The floods have now stranded a total of more than four million people and claimed 20 lives in the past week.

Air force helicopters were on stand-by to airdrop essentials as the Brahmaputra River which runs through the state swelled further.

The 2,906 km (1,816-mile) river -- one of the longest in Asia -- is usually in full flood in the summer from monsoon rains and melting glaciers as it traverses Tibet, India and Bangladesh before emptying into the Bay of Bengal.

Soldiers used rubber boats and rafts in many places to evacuate people, as communication and transport links were badly disrupted.

A Central Water Commission bulletin said all major rivers and their tributaries in Assam were flowing well above the danger mark.

The worst hit was the eastern Dhemaji district where an estimated 250,000 affected people had taken refuge in makeshift shelters on raised embankments, in government schools and offices.

A team of doctors and paramedics was sent to the flood-hit areas by boat and raft to treat people with water-borne diseases.

'We have also sent veterinarians along with adequate cattle and poultry feed in several areas,' the minister said.

The Regional Meteorological Centre here warned of more rain in the next 24 hours.