A third of Bangladesh is underwater after some of the heaviest rains in a decade, officials said.
Almost four million people have also been hit by monsoon floods in South Asia.
The monsoon – which usually falls from June to September – is crucial to the economy of the Indian sub-continent, but also causes widespread death and destruction across the region every year.
“This is going to be the worst flood in a decade,” Bangladesh’s Flood Forecasting and Warning Centre chief Arifuzzaman Bhuiyan told the AFP news agency.
The floods started late last month, and after briefly easing continued to worsen, destroying crops and driving people from their homes in several impoverished regions.
Bangladesh is crisscrossed by 230 rivers, including 53 shared with India.
The heavy rains have swollen two main Himalayan river systems – the Brahmaputra and the Ganges – that flow through India and Bangladesh.
Bhuiyan said about a third of flood-prone Bangladesh – a delta-nation crisscrossed by hundreds of rivers – was underwater, and at least 1.5 million people were affected, with village homes and roads flooded.
“The floodwater is constantly rising. Neither we nor our cattle can go out. So, I’m using a boat to go out. We’re having issues regarding food also. The cooking ovens have been flooded. Even our beds are also underwater,” Samsud Doha, farmer, said.
In north-central Bangladesh, the Brahmaputra river was almost 40cm (15.7 inches) higher than normal and threatening to burst its banks, district administrator Farook Ahmed told AFP.
Most villagers were trying to stay near their flood-damaged homes, but some 15,000 had fled severely affected areas, officials said.
Another farmer, Rabiul Islam, said: “Our homesteads have been flooded. We had a little road left which got destroyed last night. So, we’re taking away all our crops like rice and corn and other goods.”
With a 10-day forecast pointing to rising waters, Bhuiyan said if more rivers burst their banks, some 40 percent of the nation could be flooded “in a worst-case scenario”.
In Assam, northeast India, more than 2.1 million people have been affected since mid-May.
At least 50 people have died so far – 12 in the past week as floodwaters surged – with tens of thousands of mostly rural residents evacuated to relief camps, officials said.
“We have two challenges here: one is COVID-19 and another is flood,” the head of a local rescue team, Abhijeet Kumar Verma, told AFP.
In Nepal, at least 50 people have died in landslides and floods triggered by the monsoon rains, with homes swept away and roads and bridges damaged.