At least 18 people have died and millions of homes were left underwater after floods hit north-eastern India and Bangladesh.
Both countries have asked the military to help with the extreme deluge, which officials fear could intensify with further rain forecasted for the weekend.
Nine people were struck dead by lightning in Bangladesh on Friday.
In India's Assam state, at least nine people died in the floods and two million others saw their homes submerged in floodwaters, according to the state disaster management agency.
The Brahmaputra river, one of Asia's largest, breached its mud embankments, inundating 3,000 villages and croplands in 28 of Assam's 33 districts.
"The volume of rainfall has been unprecedented," said Sanjay O'Neil, an official at the meteorological station in Gauhati, Assam's capital. "We expect moderate to heavy rainfall in several parts of Assam till Sunday."
Incessant rains pummelled India for five consecutive days and saw several train services cancelled. Flood waters submerged an entire railway station in Halfong, southern Assam, and dumped mud and silt along the railway tracks.
India's army has been asked to help other disaster response agencies rescue stranded people and provide food and essentials to those whose houses are underwater.
"We are using speedboats and inflatable rafts to rescue flood-hit people," an army official said.
In low-lying Bangladesh, districts near the Indian border have been worst affected.
Water levels in all major rivers across the country are rising, according to the flood forecasting and warning centre in Dhaka, the nation's capital. The flood-prone country has about 130 rivers.
The flooding is likely to deteriorate in the worst-hit Sunamganj and Sylhet districts in the northeastern region as well as in Lalmonirhat, Kurigram, Nilphamari and Rangpur districts in northern Bangladesh, the centre said.
Flights at the Osmani International Airport in Sylhet have been suspended for three days as floodwaters have almost reached the runway, the airport manager Hafiz Ahmed said.
Bangladesh has a long and wet monsoon season and endures tropical cyclones, generated in the warm waters of the Bay of Bengal.
But as human activity heats up the planet, the south Asian country is suffering hotter summers, higher rivers and more flooding during monsoons - which are also less regular.
The disrupted weather patterns spell bad news for crop yields and disease. The charity WaterAid has warned contamination of water leads to high risk of disease outbreaks.
"Water and sanitation facilities will be destroyed, washed away," WaterAid's acting country director Hossain I Adib, said of "some of the worst flooding in decades".
Clean water will be contaminated as toilets and latrines overflow, ramping up the risk for disease outbreaks, he said, as he called for improved access to clean water.
According to the United Nation's climate science group the IPCC, about 17% of people in Bangladesh would need to be relocated over the next decade or so if global warming continues at the current rate because of rising sea levels.