Hundreds of thousands of people in Bangladesh and Myanmar were ordered Wednesday to move to safety as a cyclone barrelled towards low-lying coastal areas.

The United Nations has warned that more than eight million people could be at risk from Cyclone Mahasen, which is expected to make landfall on Thursday or Friday somewhere near the border between the two countries.

Bangladesh told hundreds of thousands of people living in low-lying areas to move to cyclone shelters, while Myanmar announced plans to move roughly 166,000 people at risk on its northwest coast.

"The military will move them to higher ground" and to emergency shelters in schools, Aung Min, minister of the Myanmar president's office, said at a news conference in Yangon.

"Some people don't want to leave. We don't want to see them die so we will move them under the law of protection from natural disasters," he said.

The cyclone appeared to have lost some of its strength as it churned northwards through the Bay of Bengal, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement released late Tuesday.

But it may still bring "life-threatening conditions" for 8.2 million people in northeast India, Bangladesh and Myanmar, it warned, adding Bangladesh's Chittagong and Cox's Bazaar areas could face the worst of a tidal surge and heavy rains.

Cox's Bazaar, a long strip of coastline, is home to ramshackle camps housing Rohingya Muslim refugees.

Local officials said 113 medical teams had been mobilised to deal with the impact of the cyclone and leave had been cancelled for all government employees.

"We've made all the preparations to face the cyclone," Mohammed Kamruzzaman, a government magistrate in charge of a Rohingya refugee camp in Cox's Bazaar, told AFP.

"We have been using loudspeakers to alert both documented and undocumented Rohingya refugees of the dangers of the cyclone.

"We've also stockpiled dry food, kept medical teams and ambulances on stand-by and shifted the sick and pregnant women from the camps to hospitals."

Cyclone Mahasen was classified as the lowest-level category one on a one-to-five scale and packing winds of up to 88 kilometres (55 miles) per hour at its centre, Shamsuddin Ahmed, deputy chief of Bangladesh Meteorological Department, told AFP.

But as a category one storm it "could unleash a storm surge of up to seven feet (two metres) high in the low-lying coastal areas" and create damage in other areas, Ahmed said.

Experts say Bangladesh is better prepared to handle cyclones than authorities across the border in Rakhine, where tens of thousands of Rohingya made homeless by communal unrest last year languish in flood-prone camps.

Myanmar state media late Tuesday said rescuers were searching for 58 missing Rohingya whose boat capsized after hitting rocks in a coastal waterway after they fled the cyclone's path to escape to higher ground.

A number of other Rohingya in Myanmar have expressed reluctance to relocate, reflecting deep mistrust of security forces following two outbreaks of violence last year that left about 200 people dead and whole neighbourhoods razed.

Rights groups have accused Myanmar security forces of complicity in the unrest.

They have also criticised Myanmar for failing to provide permanent housing sooner for displaced Rohingya, after months of warnings of the danger posed to the camps by this year's monsoon.

Myanmar said the cyclone could delay Thein Sein's planned state visit to the United States, which would be the first by a leader of the former pariah nation in almost half a century.