The United Nations says it will launch an international appeal for emergency aid to help flood-stricken Bangladesh, where 452 people have died.
The UN warns infrastructure has sustained long-term damage
Millions of people have been stranded or left homeless by floods. Two-thirds of the country - and half of the capital Dhaka - are under water.
A preliminary UN assessment says the floods have already caused extensive damage to infrastructure.
And officials have already warned that there could be a humanitarian crisis.
The rains continued on Wednesday, and forecasters warned that more could be expected across central regions.
The floods have also hit parts of India, bringing hundreds more deaths, waterborne diseases and the threat of malnutrition.
'Worst floods in years'
According to the UN, an estimated 30 million people are affected across Bangladesh, including farmland and property.
Many people are seeking shelter in public buildings, motorways, rooftops and high ground and are in need of urgent help.
The UN told the BBC it was going to prepare an international appeal for the flood victims and the post-flood recovery effort within the next five or 10 days.
Dhaka had refused to appeal for international help for the immediate relief operations, saying it could cope on its own.
But the Bangladeshi High Commissioner to London Mofazzal Karim told the BBC Dhaka had decided to call for international help.
"Right at this moment, till the water recedes, people need emergency relief, like food, medicine, water purifying tablets and things like that, and maybe clothes," he said.
The word 'monsoon' comes from the Arabic for 'season'
Describes seasonal reversals of wind direction
From April heat builds over South Asia, creating low pressure areas
Brings moisture-rich south-west winds in from the ocean
"But very shortly, there'll be a massive need for reconstruction materials, you need an enormous amount of money for reconstruction of communication systems, the damaged houses of people, the schools and so on and so forth."
While the weather in some northern parts of the country was set to improve, more rain is forecast for the central region - including Dhaka.
"I have nowhere to go. Allah seems to be unkind to me,"
Safura Begum, a 35-year-old mother living on a Dhaka pavement with her young children, told Reuters news agency.
Fifteen people were wounded in Dhaka when police opened fire to break up a fight between residents, Reuters said.
The clash happened when people in one quarter opened a sluice gate to ease flood pressure in their neighbourhood - only to inundate another.
In India, 460 people were reported to have died in Bihar state to the west of Bangladesh. Flood waters were reported to be receding.
A reported 181 people have died in the neighbouring Indian state of Assam - many from waterborne diseases, said the state health minister.
He said emergency provisions of barley and biscuits had been ordered to help thousands of infants suffering malnutrition.