Lightning strikes killed at least 15 people in eight districts in Bangladesh since Friday and four people were killed in landslides, police officials said.

The flooding in Bangladesh, described by a government expert as potentially the country's worst since 2004, was exacerbated by the runoff from heavy rain across Indian mountains. The rain continued on Saturday, with more forecast over the next two days.

"Much of the country's northeast is underwater and the situation is getting worse as heavy downpour continues," Bangladesh Chief Administrator Sylhet region Mohammad Mosharraf Hossain said.

Bangladesh and India have experienced increasing extreme weather in recent years, causing large-scale damage.

Environmentalists warn climate change could lead to more disasters, especially in low-lying and densely populated Bangladesh.

Authorities helped by the army were focused on rescuing those trapped and distributing relief.

Television footage showed Bangladesh roads and railway lines submerged with people wading through chest-high brown churning waters, carrying their belongings and livestock.

Many of Bangladesh's rivers have risen to dangerous levels, said state-run Flood Forecasting and Warning Centre Head Arifuzzaman Bhuiyan said.

In neighbouring India's northeastern state of Assam, armed forces were called in for rescue efforts after landslides killed at least nine people and displaced nearly 2 million from their homes in the last 10 days, officials said.

Torrential rains lashed 25 of the state's 33 districts for a sixth day.