At least 27 people have died and 900,000 others have been forced to leave their homes as monsoon rains swamp wide areas of the northeastern Indian state of Assam, officials said on Thursday.

A state government spokesman said 21 of Assam's 27 districts were hit by flash floods which began last weekend as the rains lashed the tea and oil-rich Indian state.

"So far, 27 people have died in separate incidents including five who were killed when their boat sank," Assam Agriculture Minister Nilamoni Sen Deka told AFP in Guwahati, the state's largest city.

He also said the Brahmaputra river was overflowing its banks in many places.

Deka said an estimated 900,000 people had been displaced from their homes due to the flooding.

"Most of the displaced people have been forced to take shelter on raised platforms and in tarpaulin tents," Deka added.

"All major rivers are running menacingly high with breaches reported in many places," added Assam health minister Himanta Biswa Sarma as rescuers set up 1,500 temporary shelters for people stranded.

In nearby Bangladesh, the death toll from a series of flash floods and landslides rose above 100 on Thursday.

The annual monsoon, crucial to India's food production and economic growth, arrived over the tropical country earlier in the month.

Indian agriculture gets 60 percent of its precipitation from the rains and a bad monsoon can spell financial disaster for the country's 235 million farmers.