A cyclone churning through the Indian Ocean towards Bangladesh and Myanmar could put the lives of 8.2 million people at risk.

The UN Office for the Coordination of Humanitarian Affairs said that Cyclone Mahasen has weakened and been downgraded to a Category 1 storm but still posed a deadly risk to millions of people in northeast India, Bangladesh and Myanmar's Rakhine state.

Mahasen is travelling northwest over the Bay of Bengal and is expected to make landfall on Friday, north of the Bangladeshi city of Chittagon near the border with Myanmar.

"The highest impact, surge and rainfall predictions are for the Chittagong and Cox's Bazaar areas of Bangladesh but there are serious concerns for Rakhine state," the UN said.

More than 140,000 displaced Rohingya Muslims live in Rakhine, many in low-lying camps along the coast that are highly susceptible to tidal surges and flooding. The refugees have fled sectarian violence that has flared with Myanmese Buddhists over the last 12 months.

The UN said that although authorities had started evacuating thousands people to safer shelters quicker action was needed.

"We are very concerned about the situation of displaced people in Rakhine state, particularly those in low-lying camps, and we are working urgently with the government to find solutions that are viable. But we are running against time and lives are at risk," said Oliver Lacey-Hall, the head of OCHA's regional office for Asia and the Pacific.

On Tuesday, a boat evacuating 200 Rohingya sank after hitting a rock. More than 50 people were still missing feared dead.

"I am deeply saddened by the loss of lives of the Myanmar coast during this evacuation," said UN humanitarian chief Valerie Amos. "It is important for the government to ensure that people can reach safety before Mahasen hits."

In 2008, Cyclone Nargis killed more than 130,000 people in Myanmar.