At least 25 people have been killed, millions of homes flooded, transport links severed and people left marooned in low-lying parts of Bangladesh over the weekend, during the country's worst monsoon floods in recent history.
In northeastern Sylhet, on the banks of the Surma River, thousands have been seeking shelter in temporary relief centres as flooding from swollen rivers has caused extensive damage to fields and property.
Flights at Osmani International Airport in Sylhet were suspended and the Sylhet Sunamganj highway was flooded.
Water levels in all major rivers across the country were rising, according to the flood forecasting and warning centre in Dhaka. Bangladesh has about 130 rivers.
In the neighbouring Indian state of Assam, at least 17 people were killed during the wave of flooding that began this month, police officials said on Sunday, local time.
Thousands of police officers and army personnel have been deployed to parts of the country to help search and rescue efforts.
About 105,000 people have been evacuated so far, but police estimated that more than four million were still stranded.
Syed Rafiqul Haque — a former ruling party politician in Bangladesh's Sunamganj district — said the country was facing a humanitarian crisis if proper rescue operations were not conducted.
"Almost the entire Sylhet-Sunamganj belt is under water, and millions of people are stranded," he said, adding that victims had no food or drinking water, and communication networks were down.
Regional officials said about 3.1 million people were displaced, 200,000 of whom were staying in government-run makeshift shelters on raised embankments or on other high land.
Last month, a pre-monsoon flash flood, triggered by a rush of water from upstream in India's northeastern states, hit Bangladesh's northern and northeastern regions, destroying crops and damaging homes and roads.
The country was just starting to recover when fresh rains flooded the same areas again this week.
Bangladesh, a nation of 160 million people, is low-lying and faces threats from natural disasters such as floods and cyclones, made worse by climate change.
According to the United Nations' Intergovernmental Panel on Climate Change, about 17 per cent of people in Bangladesh would need to be relocated over the next decade or so if global warming persisted at the present rate.