Nearly 25 million people have been displaced by flooding and 1,400 killed in South Asia as the worst monsoon rains to hit the region in decades continued to wreak havoc on Saturday.

Northern India, Bangladesh and Nepal are the worst hit countries, according to officials dealing with the crisis.

In India alone, the number of dead topped 1,100 by late Friday, the United Nations' child welfare agency said in a statement.

"According to government estimates, the cumulative number of human casualties stands at 1,103 in 138 affected districts," said UNICEF.

Northern Uttar Pradesh and Bihar states and northeastern Assam were among the worst affected, with 10 million people hit by floods in Bihar alone, according to a Press Trust of India news agency report on Saturday.

Another 5.5 million were displaced in Assam and the floods were wreaking havoc for 1.4 million people in Uttar Pradesh, officials said, with heavy rains expected to continue throughout the day.

The monsoon, which lasts from June to September, regularly brings flooding to South Asia but this year has witnessed some of the worst in living memory with the north and east particularly hard hit.

"The situation we have now is unprecedented in the past 30 years," A.K Chowdhury, Bihar chief secretary, told AFP by phone.

As many as 24 people died in the state on Friday alone, PTI said, and early estimates from officials suggest losses of 450 million rupees (11 million dollars) in the state from destroyed crops and homes.

Bangladesh said the situation this year appeared to be worse than floods in 2004 which inundated more than a third of the country.

"In the last 15 days, all major rivers rose above danger levels and their water has already inundated some 40 per cent of the country's total land area," said Saiful Hossain of Bangladesh's Flood Forecasting and Warning Centre.

Deaths from monsoon rains there topped 200 on Saturday, with at least 16 more deaths reported overnight, according to the Bangladesh food and disaster management ministry, taking the monsoon toll this year to 207.

"It's a major flood and one of the worst in years. In some places the situation is far worse than in 2004," said Hossain, referring to the floods three years ago in which 38 per cent of land was inundated, forcing millions to flee their homes.

This year, 7.5 million people have been either displaced or marooned in villages as the floods washed away or damaged some 89,000 mud-built or tin-roofed houses.

Of those, an estimated 255,000 had been moved to government shelters.

The government has mobilised thousands of military, public and private volunteers to distribute relief supplies including dry food and water purification tablets, but a lack of boats has hampered their efforts.

In Nepal, the home ministry said the toll from monsoon-triggered flooding and landslides stood at 91 on Saturday, with most of the deaths in the Terai plains region on Nepal's southern border with India.