AN acute shortage of boats is hampering efforts to relieve about 3 million people affected by a torrent of water that has devastated the impoverished state of Bihar in north India.

Two weeks ago the monsoon-swollen Kosi River broke its banks and diverted to a channel it had abandoned nearly 200 years ago near the border of Nepal and India.

More than 1700 villages in Bihar have been gradually inundated and half the districts in the state's densely populated rural areas have been affected.

A least half a million people are marooned without food, clean water and medicines.

Officials say about 70 people have died in the flooding but some non-governmental organisations have put the death toll at more than 2000. Twenty people drowned on Friday when their rescue boat capsized.

The Prime Minister, Manmohan Singh, has called the flooding a national calamity and has pledged $US228 million ($267 million) in assistance.

A relief effort is under way but the Government is under pressure to do more.

"Our greatest need at the moment is for boats," said Dr Sydney Thyle of the Emmanuel Hospital Association. "We need them to evacuate people and to transport food. The situation is mind-boggling."

UNICEF says extremely hot weather in Bihar has aggravating the suffering of the displaced population, particularly children, pregnant and breast-feeding women, and the aged.

Some victims who have made it to higher ground are being moved to temporary shelters that lack basic amenities, putting them at risk of contracting diseases.

Aid workers are bracing for an outbreak of water-borne diseases such as typhoid and cholera.

"Hygiene conditions in the camps are generally very poor with an insufficient number of toilets, resulting in open defecation," UNICEF said in a statement.

"Cases of fever and diarrhoea are being reported. Given the scorching heat, unsafe drinking water and poor hygiene conditions, cases may soon increase."