More than 1,100 people have died across South Asia since the start of the annual monsoon season in mid-June, with the region's rivers bursting their banks due to relentless heavy rains and snows melting in the Himalayas.

Britain and the United Nations have stepped up to assist local authorities, with London pledging 2.5 million dollars in aid and the world body launching emergency relief operations.

Northern India has borne the brunt of the disaster, with 1,000 people dead and more than 12 million villagers in Bihar and Assam states desperately awaiting much-needed aid -- and fears mounting that disease could soon spread.

"Whatever baby food I had is now exhausted and there is no alternative other than trying to forcibly feed my daughter boiled rice," 27-year-old Rahima Begum, whose home near the Brahmaputra River in Assam is under water, told AFP.

"My son is down with fever and diarrhoea for the last two days," said labourer Bhairab Madhab, who like Begum lives in the village of Senimari.

"Getting a doctor is a distant dream with flood waters surrounding us."

Some 5.5 million people have been displaced in Assam, while nearly seven million others are stranded in Bihar state, where 3,000 villages have been inundated, officials said.

An official in the state's hard-hit Darbhanga district said the area had been drenched with 875 millimetres (three feet) of rain in July, more than three times what it received during the entire monsoon season last year.

The Indian capital New Delhi was also battered with rain overnight, receiving 166 millimetres of rain in the last 24 hours, more than half its average for the entire month of August.