People evacuate on a truck in Sittwe in Myanmar's Rakhine state ahead of the landfall of Cyclone Mocha. AFP

A powerful storm closed in on the coasts of Bangladesh and Myanmar on Sunday, forcing the evacuation of hundreds of thousands of people to safer locations, many from frail homes in low-lying areas.

Cyclone Mocha, packing winds of up to 210 kph (130 mph), could bring sea surges of up to 12 feet (4 m) affecting more than 2 million people directly in its path, most of them in Myanmar's Rakhine and Chin states.

But many more could suffer as the storm moves inland from the Bay of Bengal, the United Nations Satellite Centre has warned.

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"Saving lives is our main priority," said disaster relief official Mijanur Rahman in neighbouring Bangladesh, which has moved about 300,000 people before the storm is expected to hit land around noon.

Aid workers are worrying about the risk to more than a million Rohingya refugees, half-a-million children among them, living in camps in the beach town of Cox's Bazar near the cyclone's path.

This satellite image shows storm Mocha intensify into an extremely severe cyclonic storm on Sunday. AP

"People at risk are in the process of being transferred to safe shelters and we are also arranging relief packages," said Farah Kabir of ActionAid Bangladesh.

Many local residents live in areas more than 3 meters above sea level, places where residents believe the storm surge cannot reach, he added.

Strong winds of 40 to 48 kilometers per hour (25 to 40 miles per hour) were blowing in the city, Tin Nyein Oo said on Sunday morning.

"The storm has not yet entered, so we don’t have much difficulty. However, there are too many people in the shelters and not enough toilets,” he added.

Lin Lin, the chairman of a local charitable foundation, said earlier there was not enough food in the shelters in Sittwe after more people arrived than expected.

A local rides a motorbike on a nearly empty road before Cyclone Mocha hits in Sittwe, Rakhine State, Myanmar. AP

In Bangladesh, weather in most areas remained sunny and humid on Sunday morning.

U.N. agencies and aid workers in Bangladesh prepositioned tons of dry food and dozens of ambulances with mobile medical teams in sprawling refugee camps with more than 1 million Rohingya who fled persecution in Myanmar.

Bangladesh issued the highest danger signal for Cox’s Bazar, home to the camps. The Bangladesh Meteorological Department warned the cyclone could cause severe damage to lives and property in eight coastal districts.

No heavy rainfall had been reported in Cox's Bazar as of Sunday morning.

Bangladesh, with more than 160 million people, has prepared more than 1,500 cyclone shelters. The navy said it's keeping ready 21 ships, maritime patrol aircraft and helicopters for rescue and relief operations.

People carry their belongings in Shahpori island, Teknaf, Bangladesh, ahead of Cyclone Mocha's landfall. AFP

Authorities in Bangladesh said heavy rains from the cyclone could trigger landslides in Chattogram and Cox's Bazar and three other hilly districts - Rangamati, Bandarban and Khagrachhari.

In May 2008, Cyclone Nargis hit Myanmar with a storm surge that devastated populated areas around the Irrawaddy River Delta. At least 138,000 people died and tens of thousands of homes and other buildings were washed away.

Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology in Pune city, said cyclones in the Bay of Bengal are becoming more intense more quickly, in part because of climate change.