The category 4 Tropical Cyclone Sidr caused a 15-foot high tidal surge and forced 3.2 million people to evacuate, officials and aid agencies told Reuters. Most of the deaths were caused by flimsy homes made from bamboo and tin collapsing or being crushed.
The U.N. Office for the Coordination of Humanitarian Affairs told reporters in Geneva that 1,000 fishermen are missing.
"Significant damage is expected. However, information collection on casualty and damage figures is still very much in the early stages," spokeswoman Elisabeth Byrs said.
The Bangladesh government initially put the death toll at 242, but acknowledged that making a count was difficult and that the numbers were expected to rise. The southern districts of Barguna and Jhalokati were hit the worst.
"I cannot describe how devastating it was. It was like doomsday, the most frightening five hours of my life. I thought I would never see my family again," businessman Mollik Tariqur Rahman told the AFP.
The government's disaster agency is also concerned about damage to the recent crop harvest.
By Friday morning, the cyclone was downgraded to a tropical storm, but some inland areas could still be at risk.
CNN meteorologist Kevin Corriveau said possible rainfall in the mountains would swell rivers, and by Sunday night or Monday the run-off could reach already flooded lands in the country.
Many of the country's 140 million people live on low-lying river deltas that are especially vulnerable to tidal surges and flooding.
Members of the Bangladesh Navy have started search and rescue efforts.
U.S. military officials said the Department of Defense was ready to dispatch Navy vessels and 3,500 Marines to the region, according to CNN.
Bangladesh and northeastern India are regularly hit by cyclones that form in the Bay of Bengal. A cyclone in 1970 left 500,000 people dead in Bangladesh.