The Bangladesh Meteorological Department said in a bulletin that at 0600 GMT the storm was about 1,300 km (800 miles) south of Chittagong port with a maximum wind speed of 88 km (55 miles) per hour. It said wind speeds were likely to more than double as the storm intensified and forecast landfall on the Bangladesh coast for Wednesday or Thursday.
"The evacuation will start when the storm comes dangerously close to the shores," a disaster management official said, adding that the army and police were ready to help.
Storms batter the poor south Asian country every year, killing hundreds of people. The latest warning came 37 years to the day after the most severe storm to hit Bangladesh, on Nov. 12, 1970, which killed more than half a million people.
(Reporting by Nizam Ahmed, Writing by Anis Ahmed, Editing by John Chalmers)