Extreme flooding has claimed the lives of over 1000 people in South Asia and left an estimated 20 million marooned, in what officials said today was the worst monsoon season the subcontinent had seen in 30 years.

UNICEF went further, saying India, Nepal and Bangladesh were experiencing "the worst flooding in living memory."

"The sheer size and scale of the flooding and the massive numbers of people affected poses an unprecedented challenge to the delivery of desperately needed humanitarian assistance," the UN agency said.

In India, the floods claimed 21 victims overnight in three severely afflicted eastern districts according to Uttar Pradesh relief commissioner Umesh Sinha. The latest deaths bring the toll to 1,028 in India alone, according to official and media reports.

Floods triggered by monsoon rains inundated swathes of northern and eastern India, with the worst-hit states Uttar Pradesh, Bihar, West Bengal and Assam. In Uttar Pradesh, some 1,650 paramilitary and army personnel have been deployed along with civilian rescue teams to help an estimated 1.4 million people cut off by the floods, Mr Sinha said. Many rivers in the state have burst their banks, and further downpours are expected in the next 24 hours.

In neighbouring Bihar state, chief minister Nitish Kumar described the situation as "grim", reporting more than seven million people stranded amid overflowing rivers. In the worst affected district, Darbhanga, it was impossible to reach all parts by boat so airdrops of relief material were being planned, he continued.

A.K Chowdhury, the chief secretary in Bihar, said: "The flood situation is very very serious, the situation we have now is unprecedented in the past 30 years." In addition to the human cost, the floods were damaging the state's economy, he said, reporting over 630,000 hectares of crops destroyed with estimated losses of 450 million rupees (£5.5 million).

India's neighbours too are suffering badly. Nepal's home ministry reported a death toll of 87, while Bangladesh saw 11 further deaths overnight taking its total to 191.

Tapan Chowdhury, the Bangladeshi disaster management minister, said around 6.9 million people were either displaced or marooned in villages, with thousands of army and civilian personnel engaged in emergency relief work. Officials said a lack of boats was hindering efforts so rescue teams were fashioning rafts out of banana trees.

In the district of Sirajganj, villagers told how they had fled their homes as the floods engulfed the land.

"The floods have taken away all I had," said Rahmat Sheikh, 40, one of 2000 people displaced from a single village. "Rice paddies in the field, two cows and my house all are gone. I don't know how we will now survive."

Meanwhile in Nepal, some 32 out of the Himalayan nation's 75 districts have been affected. Biratnagar, a border town close to India, reported 223 millimetres (8.8 inches) of rain in the last 24 hours.

The ferocity of this year's monsoon will add fuel to claims that global warming is already inducing extreme weather effects. Several other countries, including Indonesia, China and to a lesser extent Britain, have all experienced unusually devastating floods in the past month. On the subcontinent, experts have for some years associated the increasing severity of the annual monsoon with glacial melt in the Himalayas caused by rising temperatures.

There was hope today as the storm clouds began to part and a Central Water Commission bulletin reported receding water-levels in the major rivers and their tributaries. But with millions of people still to be reached and the prospect of further deluges before the annual monsoon season ends in September, the disaster is far from over.

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