Floods are a regular menace to millions of people in low-lying Bangladesh, but experts say climate change is increasing their frequency, ferocity and unpredictability, AFP reported.
Relentless downpours over the past week have inundated vast stretches of Bangladesh's northeast, with troops deployed to evacuate households cut off from neighbouring communities.
Schools have been turned into relief shelters to house entire villages inundated in a matter of hours by rivers that suddenly burst their banks.
"The whole village went under water by early Friday and we all got stranded," said Lokman, whose family lives in Companiganj village.
"After waiting a whole day on the roof of our home, a neighbour rescued us with a makeshift boat. My mother said she has never seen such floods in her entire life," the 23-year-old added.
Lightning triggered by the storms has killed at least 21 people around the South Asian nation since Friday afternoon, police officials told AFP.
Among them were three children aged between 12 and 14 who were struck by lightning on Friday in the rural town of Nandail, said Mizanur Rahman, local police chief.
Another four people died when landslides hit their hillside homes in the port city of Chittagong, police inspector Nurul Islam said.
At least 16 people have been killed since Thursday in India's remote Meghalaya, the state's Chief Minister Conrad Sangma wrote on Twitter, after landslides and surging rivers that submerged roads.
Next door in Assam state, more than 1.8 million people have been affected by floods after five days of incessant downpours.
Assam Chief Minister Himanta Biswa Sarma told reporters he had instructed district officials to provide "all necessary help and relief" to those caught in the flooding.
Flooding in Bangladesh worsened on Saturday morning after a temporary reprieve from the rains the previous afternoon, Sylhet region chief government administrator Mosharraf Hossain said.
The flooding forced Bangladesh's third-largest international airport in Sylhet to shut down on Friday.
Forecasters said the floods were set to worsen over the next two days with heavy rains in Bangladesh and upstream in India's northeast.