The death toll from monsoon floods in India, Bangladesh and Nepal has climbed above 1,200, as rescue workers scramble to provide aid to millions of people stranded by the worst such disaster in years.

All three countries suffer frequent flooding during the June-September monsoon season, but international aid agencies say things are worse this year with thousands of villages cut off and people deprived of food and clean water for days.

Indian Prime Minister Narendra Modi conducted an aerial survey of flood-hit Bihar state on Saturday and has pledged a relief fund of $78m.

Government officials in India's eastern state of Bihar told Reuters news agency on Friday that at least 379 people had been killed over the past few days, with thousands sheltered in relief camps away from their inundated homes.

"This year farming has collapsed due to floods and we will witness a sharp rise in unemployment," said Anirudh Kumar, a disaster management official in Patna - the capital of poor Bihar state known for mass migration to cities in search of jobs.

In neighbouring Uttar Pradesh, at least 88 people were killed when floods swamped nearly half of the vast state of 220 million people.

Rajan Kumar, a federal interior ministry official in New Delhi overseeing the rescue and relief operations, told Reuters news agency that at least 850 people had been killed in six flood-affected states in the past month.

"A second wave of floods led to widespread destruction," he said. "We will have to provide immediate rehabilitation aid to help millions affected directly."

'Left with nothing'

In Nepal, 150 people have been killed and 90,000 homes destroyed in what the United Nations is calling the worst flooding there in a decade.

In Bangladesh, at least 134 people have died and more than 5.7 million were affected directly as monsoon flooding submerged more than a third of the low-lying and densely populated country.

IN PICTURES: Deadly floods hit South Asian states

Crops on 10,583 hectares of land have been washed away while another 600,587 hectares of farmland have been partially damaged, according to the disaster ministry, in a big blow to the farm-dependent country which lost around one million tonnes of rice in flash floods in April.

"I could not find a single dry patch of land," said Matthew Marek, the head of disaster response in Bangladesh for the International Federation of Red Cross and Red Crescent, who made an aerial assessment of the worst affected parts of the country.

"Farmers are left with nothing, not even with clean drinking water."