KHULN, Bangladesh: Several million people sheltered and prayed on Wednesday as one of the fiercest cyclones in moved roared towards Bangladesh and eastern India, with forecasts of a deadly storm surge.

A few hours ahead of the predicted arrival of Amphan, the Bangladeshi Red Crescent reported the first death, after one its volunteers drowned when strong winds capsized a boat.

Authorities have evacuated low lying areas in the storm's projected trail of destruction, but their task is complicated by the need to follow precautions to prevent the spread of coronavirus.

Amphan is only the second "super cyclone" to form over the Bay of Bengal since records began, and the first since 1999.

At midday (1.30pm in Thailand) the vast vortex -- visible from space -- was 125 kilometres offshore and forecast to hit the coast in the afternoon or evening.

Gusts of 185 kilometres per hour were predicted when it does with a storm surge of several metres, forecasters said.

Such walls of water can travel several kilometres inland, and are often the main killers in any cyclone, typhoon or hurricane.

The Indian met office warned of possible flying objects, "extensive" damage to communications and power lines, and trees being ripped out of the ground by the wind.

Kolkata was battered by heavy rain and the Hooghly river was rising, while in the coastal resort of Digha, large waves were pounding the shore.