A cyclone battered coastal Bangladesh on Saturday, killing at least 21 people and injuring many more, but has now weakened into a depression that the weather office said could still bring brief periods of violent wind or rain.

Authorities in low-lying Bangladesh have moved about 500,000 people into 3,500 shelters, the disaster minister said. Cyclone Roanu has killed people in house collapses, landslides and a storm surge that broke embankments at two places in the Chittagong port city in the southeast.

"We've shifted most of the people who are vulnerable," said Disaster Management and Relief Minister Mofazzal Hossain Chowdhury Maya, adding that people were working "all-out" to contain and tackle the damage.

Several people were injured after gusty winds damaged houses and shops and uprooted trees and electric poles. Some places were inundated by a storm surge of 3 to 4 feet (a metre or more) above normal tide height, officials said.

Authorities were also evacuating people from hilly parts of Chittagong in case persistent rain triggers more landslides.

Officials suspended flights at Chittagong airport while the Bangladesh Inland Water Transport Authority restricted movement of all ships and ferries.

The weather office, in its final update for the day, advised fishing boats and trawlers to remain in shelter until Sunday noon.

Bangladesh is frequently battered by such storms. More than 3,000 people were killed by Cyclone Sidr in 2007 and around 200 lives were lost to Cyclone Aila in 2009.-Reuters