More than two million people are now in need of food, clothing and shelter.
Flooding in Bangladesh left three million people stranded in 25 of the country's 64 districts. At least 54 people have been killed. At least 47 people have died in southern Nepal and five in Pakistan's northwest.
More rain is forecast in Bangladesh and northeastern India in the next few days. The monsoon rains typically end in September.
An overflowing river spilled into an Indian city of half a million people yesterday, and Indian and Nepalese helicopters rushed supplies to thousands of stranded people as the death toll across South Asia from monsoon flooding rose to 326.
The fatalities were the result of building collapses, drownings, electrocution and waterborne diseases since the torrential rain began in mid-June across India, Pakistan, Bangladesh and Nepal.
With more rain forecast in Bangladesh and India's northeast, flooding is likely to worsen, relief officials said. The most deaths - 176 - have been in India.
Eight people were reported dead yesterday in the East Champaran district of India's eastern Bihar state when a boat ferrying them to safety across the Burhi Gandak River capsized, said Upendra Sharma, deputy secretary in the relief and rehabilitation department.
In neighbouring Assam state, where the Brahmaputra River burst its banks and poured into the capital, Gauhati, four more people were killed when a banana tree raft on which they were travelling to higher ground capsized near Hajo, 18 miles north of the capital.
"The Brahmaputra has entered Gauhati city at several places, displacing several hundred people," said local administrator Absar Hazarika.
Assam authorities have asked the Red Cross for food, drinking water, tents, clothes and mosquito nets.
They have also requested doctors, nurses and medicine.
At least five million people have been displaced or marooned in their homes in Assam, and Indian air force helicopters were dropping supplies to some of those stranded, the government said.
In neighbouring Bangladesh, the rain has engulfed 25 of its 64 districts since late June, stranding more than three million people in their flooded homes and killing 55, most of them children who drowned.