The UN children's fund (Unicef) and the World Health Organisation said that stagnant floodwaters were breeding grounds for diarrhoeal and waterborne diseases, including cholera as well as insect-borne diseases such as malaria and dengue fever.

The plight of more than 28 million flood victims in India, Bangladesh and Nepal is amplified by the difficulty in airlifting vital clean water supplies to such huge numbers of people, they added.

"Entire villages are days away from a health crisis if people are not reached in the coming days," Unicef's health chief in India, Marzio Babille, said in a statement.

"Children who make up 40 percent of South Asia's population, are particularly susceptible," he added.

The UN agencies said water sources in the affected areas are either contaminated or still submerged more than a week into the flooding caused by exceptionally heavy monsoon rains. Many people were relying on drinking dirty surface water.

"It's the distribution of drinking water that's a problem, you can't airdrop plastic bottles of water," Unicef spokeswoman Veronique Taveau told journalists.

"Airlifts are probably the only way to reach people at the moment. This is what is crucial to organise at this very moment," she added. National authorities have been ferrying supplies to remote communities by helicopter.

The WHO said it had been informed about outbreaks of diarrhoeal illnesses in recent days and warned of the need to maintain surveillance for waterborne diseases.

"We know that these are the biggest killers in such situations," said WHO spokeswoman Fadela Chaib.

India is the worst affected country, with some 20 million people in Assam, Bihar and Uttar Pradesh states hit by flooding and often forced to flee to higher ground, according to the UN agencies.

Some eight million people are affected in Bangladesh and another 300,000 people in southern Nepal, the UN said.