Three days of relentless downpours sparked flash floods and landslides that have killed at least 70 in Nepal and 73 across northern and eastern India.
Around 200,000 are living in emergency camps in Assam in northeast India, which suffers frequent flooding during the annual monsoon rains.
Another 15,000 have had to leave their homes in the eastern state of Bihar, which borders Nepal and where one official said seven rivers in Bihar were at danger levels.
Huge swathes of the state were submerged in 2008 when a river burst its banks across the border in Nepal, with the two countries trading blame for the disaster.
Massive landslide
A massive landslide in the mountainous northern state of Himachal Pradesh swept two passenger buses off a hillside and into a deep gorge, killing at least 46 people on Sunday.
In Nepal, over 48,000 homes have been totally submerged by the floods and 21,000 people displaced, according to police.
As emergency workers struggled to reach far-flung areas, the country's home ministry said another 47 people were missing, presumed dead.
The Nepal Red Cross warned that shortages of drinking water and food could create a humanitarian crisis in the impoverished Himalayan country.
"In many parts of the country there is a scarcity of safe drinking water creating a high risk of health hazards," spokesman Dibya Raj Poudel told AFP.
"Several villages and settlements are unreachable. Telecommunications, mobile phones are still not working so it difficult to give a full assessment."
'Inundated'
The southern lowlands of Nepal known as the Terai have so far borne the brunt of the monsoon deluge.
The Terai is Nepal's most fertile region and the economic toll is expected to be severe.
"We are getting reports that about 70 per cent of agriculture area in the Tarai is inundated," said Shankar Sapkota, senior agricultural economist with the government.
"Paddy fields, vegetable plantation and fish farms have been affected but right now we cannot confirm the extent of damage."
The rains are now expected to shift westwards and authorities in Nepal have begun evacuating 74,000 people from the area expected to be hit.
Nearly 150 people have been killed in Nepal since the beginning of the rainy season in late June.
Hundreds have died in torrential rain, floods and landslides in neighbouring India during the monsoon, which hits the country's southern tip in early June and sweeps across the nation, lasting into September.
In the northern Indian state of Uttarakhand – which borders Nepal – three people were killed in a landslide late on Sunday triggered by heavy rains, local police official Ajay Joshi told AFP.
