Almost 20 million people have been displaced as some of the worst floods for years have hit a wide swathe of northern India, Bangladesh and Nepal.
Roads have been washed away and hundreds of villages have been cut off by swollen rivers.
A BBC correspondent in the Indian state of Assam says the air force is organising food drops, but they are nowhere near enough.
Almost 200 people have died in the floods in the last few days.
In Bangladesh thousands of families are on the move in search of higher ground.
THE AFFECTED AREAS
India: 125 people killed and 12 million stranded, mostly in Bihar, Uttar Pradesh and Assam
Bangladesh: 64 people have been killed and seven million are marooned
Nepal: Thousands of people displaced in the south
Hundreds of thousands of people across the affected area are at risk from hunger and disease.
The BBC's South Asia correspondent, Damian Grammaticas, says that food, clean drinking water and medical aid are the priorities, but just a fraction of those who need them are receiving supplies as aid agencies and government teams struggle to get through.
It has been raining heavily in the region for 20 days. Some rivers have seen their levels rise nine or 10 metres, swamping embankments and submerging huge tracts of land.
Initial government figures say at least 125 people have been killed in India in recent days, and around 64 in Bangladesh.
It means that more than 1,000 people have died across South Asia since the start of the annual monsoon in mid-June.
The number of dead is expected to rise sharply as news comes in from more remote areas. An estimated five million hectares of farm land is under water.
In some areas, the floods are being called the worst in living memory.
THE ASIAN MONSOON
Monsoon winds blow north-easterly for one half of the year, and from the south-west for the other half
South-westerly winds bring the heavy rains from June to Sept
Winds arrive in southern India six weeks before the north west
Annual rainfall varies considerably
The bulk of the rain is now expected in central India, a region which has so far received a weaker monsoon. Already parts of the state of Maharashtra are waterlogged.
In Assam, in north-eastern India, three feet of rain fell in July.
People in the state have clashed with police in their desperation for food, shelter and medicine.
In Uttar Pradesh the army was called in to evacuate 500 villages.
The two worst affected districts are reported to be Gorakhpur and Kushinagar, although water levels in major rivers there are reported to have stopped rising for the moment.
At least 121 relief camps and 34 cattle camps have been set up in the flood-affected areas of Bihar.
Many roads and bridges in the states of Bihar and Assam have been damaged, making it harder for the authorities to get relief material to those affected.
Families have been forced to wade through waist-deep waters
Officials in the Bangladeshi district of Sirajganj are struggling to reach some of those marooned by the rising waters.
The BBC's John Sudworth - in Sirajganj - says that a lack of boats is hampering the relief effort, so rafts are being constructed from banana trees.
Furthermore, there are food shortages.
"Everything is underwater," says mother of seven Musamat Manwara Khatoum as she stands knee-deep in water.
"We've lost our crops, there's nowhere to put the children down, not even a place to cook."
Forecasters in the area say some river levels are still rising but the situation is not yet as severe as the flooding in 2004 in which 700 people lost their lives and millions had to leave their homes.
In Nepal, several rivers that flow down from the Himalayas have burst their banks in the heavily populated and low-lying Terai region that borders Bihar and Uttar Pradesh.
The country's Red Cross says a quarter of a million people have been affected by rains.
There have been deadly landslides in the highlands and floods have hit dozens of districts in the low-lying Terai region.