DHAKA (NYTIMES) - Across southern Asia, more than four million people have been hit hard by monsoon floods that have destroyed homes and structures, drowned entire villages and forced people to crouch on rooftops hoping for rescue.

The monsoon season - usually June to September - brings a torrent of heavy rain, a deluge that is crucial to South Asia's agrarian economy.

But in recent years, the monsoon season has increasingly brought cyclones and devastating floods, causing the internal displacement of millions of people in low-lying areas, particularly in Bangladesh.

Last year, at least 600 people were killed and more than 25 million affected by flooding because of the torrential monsoon rains in Bangladesh, India, Myanmar and Nepal, according to the United Nations. And in 2017, more than 1,000 people died in floods across South Asia.

Rainfall has been heaviest this year in north-east India, Bangladesh, Bhutan, Myanmar and Nepal, according to the South-east Asia Flash Flood Forecast System, which is affiliated with the United Nations.

Mr Enamur Rahman, the Bangladeshi minister for disaster management, said the inundations were the worst in decades and that hundreds of thousands of families had been marooned, forcing the authorities to open more than 1,000 emergency shelters.

"We are fighting the catastrophe with every possible resource available," Mr Rahman said. "It seems rains and floods will be prolonged this year."

India has also suffered immensely. Floods have swept across the states of Assam, Bihar, Odisha, West Bengal and other areas in the eastern part of the country.

The authorities have said that at least 85 people have died, with more than three million affected by the deluge.

In the north-eastern state of Assam, Kaziranga National Park, a World Heritage site that is home to the one-horned Indian rhinoceros, a species listed as vulnerable by the WWF, has been completely inundated. Officials said that more than 50 animals died in the flooding, though some wildlife had been rescued.

With more than a dozen rivers and tributaries swelling above the danger mark, rescue operations have been carried out in at least 22 districts across Assam.

In Nepal, 67 people have died and 40 others are missing, according to the National Emergency Operation Centre.