Governments and aid agencies are stepping up efforts to help millions of people affected by devastating floods in South Asia.
The charity Save the Children has launched an appeal for donations amid warnings of food and medical shortages.
At least 200 people have died in some of the worst floods to hit northern India, Bangladesh and Nepal in years.
In Nepal, the situation is beginning to improve, with floodwaters receding and clean-up operations under way.
But across the region entire villages have been marooned, with countless communities cut off from dry food and clean drinking water.
The BBC's South Asia correspondent, Damian Grammaticas, says governments in India and Bangladesh are trying to deliver supplies, but many areas cannot even be reached by boat, so air drops are the only way.
Helicopters dropped food to some two million people in 2,200 villages in eastern Uttar Pradesh, the Associated Press news agency reported.
Northern Uttar Pradesh and Bihar states and north-eastern Assam are among the worst affected areas of India.
"The situation we have now is unprecedented in the past 30 years," AK Chowdhury, Bihar chief secretary, told AFP news agency.
THE AFFECTED AREAS
India: 12 million people stranded, mostly in Bihar, Uttar Pradesh and Assam
Bangladesh: Seven million people marooned
Nepal: Thousands of people displaced in the south
In Bangladesh, some 41 of the country's 64 districts have been affected by the floods, with parts of the capital, Dhaka, under water.
Thousands of Bangladeshi military personnel and volunteer workers are helping distribute relief supplies and sanitation equipment, but the operation has been hampered by a lack of boats, AFP said.
The United Nations children's fund Unicef says the sheer scale of the problem poses an unprecedented challenge.
"Shelter, access to fresh water, food, emergency medical supplies and basic household items are urgently required - especially given the loss of infrastructure including basic health units and hospitals," it said in a statement.
Save the Children launched an appeal for cash to help "children and their families whose lives have been turned upside down by the flooding".
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
"Homes have been swept away, water supplies have been contaminated and they are exposed to the dangers of water-borne diseases."
The charity says almost 14 million people in India and another seven million in Bangladesh are feeling the impact of the floods.
Our correspondent in Delhi says flood waters are continuing to rage across a huge area.
People have been left camping on their own rooftops, stranded in trees or shifting their families, children and livestock on to higher ground to wait out the floods, he adds.
Initial government figures say at least 125 people have been killed in India in recent days, and around 64 in Bangladesh.
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.
A quarter of a million people have been affected in Nepal, but after weeks of heavy rain the situation there is beginning to improve, says the BBC Mark Dummett in Janakpur, near the Indian border.
The city of Hindu temples and bicycle rickshaws is returning to normal, he says.
Life though is still hard in much of the surrounding countryside, where bridges and roads have been washed away and the only way to reach some districts is by helicopters, our correspondent adds.
Medical teams and food supplies have only just begun to reach them.