A cyclone slammed into Bangladesh and India on Monday, damaging homes and bursting dams that left at least 300,000 people marooned by flooding and 15 people dead, officials and reports said.
A military-led rescue and relief operation was under way in Bangladesh, where the food and disaster minister said five people had been confirmed dead.
"Five people died in the cyclone and we are still getting reports of the damage. The government has deployed the army, navy, coastguard and police to assess the situation," spokesman Abdur Razzak told reporters.
In India, 10 people were killed, according to reports.
The cyclone hit the crowded state capital of Kolkata, bringing down trees, electricity towers and smashing cars, witnesses said.
Five of the dead were in Kolkata, West Bengal Chief Minister Buddhadeb Bhattacharjee said, while state authorities appealed to the military for help and state finance minister Ashim Dasgupta warned the death toll would rise.
Razzak said Cyclone Aila made landfall between Bangladesh's Khulna district and Sagar Island in India's West Bengal state, packing winds of up to 90 km an hour and unleashing a surge as high as four metres.
Many villages were submerged, according to officials, he said.
Around 100 villages along India's eastern coast were under water, according to government minister Kanti Ganguly.
Officials had earlier evacuated around 100,000 people from the less populated Indian side of the Sundarbans mangrove forest.
In Bangladesh, 400,000 people were moved from five districts to cyclone shelters and schools before the storm hit, with another 300,000 others stranded by flooding along the coast.
"Dozens of levees have burst due to a tidal water surge as high as 12 feet, leaving at least 200,000 people stranded," area chief Kazi Atiur Rahman told news agency AFP.
"The situation is very grave here. Hundreds of mud-built houses and bamboo shacks have been washed away. Even the office where I am working is five feet under water," he said.
In Koyra, another area close to the Sundarbans, about 100,000 people were marooned after a nearby dam burst, sub-district chief Arif Pasha said.
The surge has also flooded the town of Barguna, in the district of the same name, with other low-lying areas and islands in the vicinity also under water after levees overflowed, a local official said.
District chiefs in neighbouring Satkhira, Patuakhali and Bagerhat said that they evacuated another 230,000 people as a tidal surge along with strong wind and heavy rains hit the coastal villages.
In November 2007, more than 3,500 people were killed when Cyclone Sidr hit the same districts, the second-strongest storm recorded in Bangladesh.
The low-lying country frequently experiences tropical storms and cyclones during the monsoon season. The first of the season made landfall last month causing little damage.
In 1970, some half a million people died when a cyclone hit the impoverished country, while an estimated 138,000 people died as a result of a cyclonic surge in 1991.
The lower death tolls in 1991 and 2007 were attributed to a network of cyclone shelters and a warning system introduced after the 1970 disaster.