India and Bangladesh evacuated around half a million people as coastal areas prepared for the most powerful storm in a decade - expected to hit shores on Wednesday. 

The authorities' task to save lives was complicated by ongoing efforts to curb the coronavirus pandemic and enforce social distancing to avoid a surge of infections.

Many thousands of migrant workers are on the roads trying to get home from big cities after a nationwide lockdown destroyed their livelihoods.

Approaching from the Bay of Bengal, super cyclone Amphan was expected to hit the coast of eastern India and southern Bangladesh with winds gusting up to 185 kmh - the equivalent of a category 5 hurricane.

The Indian weather department forecast a storm surge of two storey high waves that could swamp mud dwellings along the coast, uproot communication towers and inundate roads and rail tracks.

There will be extensive damage to standing crops and plantations in the states of West Bengal and Odisha while large boats and ships could get torn from their moorings, the weather service warned.

Authorities were hastily repurposing quarantine facilities for the looming cyclone soon after easing the world's biggest lockdown against the virus, which in India is reported to have infected more than 100,000 people and killed 3,163.

Neighbouring Bangladesh, where the cyclone threatens the worst storm in about 15 years along a low-lying coast, was moving people to higher ground and urging use of masks against the virus, which has caused 20,995 infections and 314 deaths.

"We have taken necessary steps so that people can maintain distance and wear masks," said Enamur Rahman, the junior minister for disaster management, adding that 12,000 cyclone shelters were set up to accommodate more than five million people.

Aid workers have stockpiled emergency items such as food, tarpaulins and water purification tablets.

"We are really very worried," said Haiko Magtrayo, a worker of the International Committee of the Red Cross based in the nearby town of Cox's Bazar.

Hundreds more Rohingya, rescued from boats adrift in the Bay of Bengal, are living on the flood-prone island of Bhasan Char.

Health facilities in the Cox's Bazar camps recorded the first two positive cases last week - in a refugee and in a local Bangladeshi.

Limited space and poor housing leaves refugees extremely vulnerable, Snigdha Chakraborty, an official of Catholic Relief Services, said in a statement.

"There are no evacuation shelters in the camps and we are worried about damage from flooding, wind and risk of Covid-19," she added.

India, with a coastline stretching 7,516km, is struck by more than a tenth of the world's tropical cyclones.