Torrrential rain brought by cyclone Remal caused a collapse in a stone quarry in India's remote northeastern state of Mizoram, killing at least 12 people, while harsh weather hampers efforts to rescue those trapped, authorities said on Tuesday.

Weather authorities said the powerful cyclone had weakened into a depression after devastating regional coastlines the previous day, killing at least 16 and cutting power to millions in parts of eastern India and neighbouring Bangladesh.

Bitter weather was holding up rescue efforts in Melthum, the site of the quarry outside the state capital of Aizawl, a state disaster management official said.

"There have been incessant rains in the wake of cyclone Remal, which led to the quarry collapse," the official told Reuters, speaking on condition of anonymity.

There were no immediate details of the number trapped.

The heavy rain in Mizoram, which borders Bangladesh, also unleashed landslides in the region.

Authorities in India's eastern state of West Bengal were working to restore electricity lines in the worst-affected areas, after Remal stripped power lines and uprooted trees.

Weather experts in Bangladesh said the deadly cyclone was one of the quickest-forming and longest-lasting they'd experienced, blaming climate change for the shift.

"In terms of its land duration, it is one of the longest in the country's history," Azizur Rahman, director of the state-run Bangladesh Meteorological Department told AFP, adding it had battered the country for more than 36 hours.

In contrast, cyclone Aila, which hammered Bangladesh in 2009, lasted around 34 hours.

Slow-moving – and therefore longer-lasting – storms bring greater destruction.

"I've seen many storms in my life but nothing like this cyclone," said Asma Khatun, an 80-year-old widow who lives with her son, a fisherman in Bangladesh's hard-hit coastal town of Patuakhali.

"Before, the storm came and went away... now it doesn't seem to go away. The incessant pouring and heavy wind kept us stuck for days."

Rahman said the cyclone triggered massive rains, with some cities receiving at least 200 millimetres.

Storm surges breached multiple embankments, meaning seawater flooded into farmland, damaged freshwater fish farms common along the coast, or corrupted drinking water.

Bangladesh's state minister for disaster Mohibbur Rahman said 3.75 million people had been affected by the cyclone, more than 35,000 homes were destroyed, and another 115,000 damaged. (Reuters/AFP)