Almost 4 million people have been hit by monsoon floods in South Asia, officials said Thursday, with a third of Bangladesh already underwater from some of the heaviest rains in a decade.

The monsoon, which usually falls from June to September, is crucial to the economy of the Indian sub-continent, but also causes widespread death and destruction across the region each year.

"This is going to be the worst flood in a decade," Bangladesh's Flood Forecasting and Warning Centre chief Arifuzzaman Bhuiyan claimed.

The heavy rains have swollen two main Himalayan river systems, the Brahmaputra and the Ganges, that flow through India and Bangladesh.

Bhuiyan said about a third of flood-prone Bangladesh, a delta-nation crisscrossed by hundreds of rivers, was underwater, and at least 1.5 million people were affected, with village homes and roads flooded.

In north-central Bangladesh, the Brahmaputra river was almost 40 centimetres (15.7 inches) higher than normal and threatening to burst its banks, district administrator Farook Ahmed said.

Most villagers were trying to stay near their flood-damaged homes, but some 15,000 had fled severely affected areas, officials said.

With a 10-day forecast pointing to rising waters, Bhuiyan said if more rivers burst their banks some 40 percent of the nation could be flooded "in a worst-case scenario."

In Assam, northeast India, more than 2.1 million people have been affected since mid-May.

At least 50 people have died so far, 12 in the past week as floodwaters surged, with tens of thousands of mostly rural residents evacuated to relief camps, officials said.

Emergency services personnel wore head-to-toe bright-orange suits to protect themselves from the floods and coronavirus, which has infected almost 17,000 people in Assam, as they used boats to reach stranded villagers.

"We have two challenges here, one is COVID-19 and another is (the) flood," the head of a local rescue team, Abhijeet Kumar Verma said.

In Nepal, at least 50 people have died in landslides and floods triggered by the monsoon rains, with homes swept away and roads and bridges damaged.

"We are distributing food and relief goods from helicopters to about 300 displaced families after the roads were blocked by floods and landslides," district official stated.