Officials in Bangladesh say flood problems have worsened in central areas as monsoons continue to disrupt large areas of Bangladesh and India.
A mother and child are hoisted to safety in Bihar
The United Nations children's fund (Unicef) says action is needed to prevent epidemics of dysentery and cholera in affected areas.
In the Indian state of Assam the chief minister says there is an urgent need for more relief supplies.
Millions of people in the region have been affected by the monsoon floods.
Scores of people have died in India, Bangladesh and Nepal, although precise figures for the dead are hard to establish.
Flood warning officials in Bangladesh say waters rolling down from the north of the country have aggravated problems in central areas.
They are warning that more areas around the capital, Dhaka, could be flooded at the weekend.
Countless villages have been deluged
Thirty-three of Bangladesh's 64 districts have been affected, leaving at least three million people homeless or marooned.
The situation in north and north-eastern districts has improved.
Unicef says the first consignment of one million water purification tablets should be delivered to Bangladesh on Saturday.
"We want to avoid epidemics which in such cases can spread very rapidly especially with temperatures now above 35 degrees Celsius," Unicef spokesman Damien Personnaz said.
Across Bangladesh's eastern border in the Indian state of Assam, the authorities say nearly nine million people have been affected by the floods.
"We're trying to meet the needs of the people but we require urgent help in the form of food supplies, medicines, relief materials besides voluntary health care workers to reach out to the people," Assam's Chief Minister Tarun Gogoi told the AFP news agency.
In the Indian state of Bihar there have been more reports of local people looting government food stores.
The authorities there say 11 million people have been hit by the floods.
Floods and landslides are common in South Asia during the monsoon season when annual rains combine with melting snow from the Himalayas.
Have you been affected by the floods? Send us your experiences using the form below. If you have any pictures of the floods you can send them to firstname.lastname@example.org
Disclaimer: The BBC may edit your comments and cannot guarantee that all e-mails will be published.