The storm was also likely to trigger tidal surges as high as six metres that would threaten low-lying areas in 15 coastal districts, he said.
The country's meteorological department issued a bulletin on Wednesday putting the country's three major maritime ports - Chittagong, Mongla and Cox's Bazar - on the highest level of alert.
"It [the cyclone] is likely to intensify and move in a northerly direction, and may cross the Khulna-Barisal coast by Thursday evening," the meteorological department warned.
Volunteers helped evacuate thousands of people on Wednesday from coastal areas as ships were warned to return to shore.
Thousands of villagers moved to cyclone shelters, concrete buildings on raised pilings, or sought refuge inside "mud forts" - mud walls built along the coast to resist tidal surges.
Schools, mosques and other public buildings have also been turned into makeshift shelters.
About 3.2 million people were expected to be moved later on Thursday, said Ali Imam Majumder, a senior government official.
At least 620,000 people are already in official shelters, where they were being given emergency rations, he said.
Tanvir Chowdhury, editor of News from Bangladesh, told Al Jazeera from Dhaka: "People are worried although it is not the first time Bangladesh is facing a cyclonic storm in the bay. It is an every-year thing but this time it is serious and normally you do not get a number 9 or ten signal.
"Last time was in 1970 when half a million people perished in the coastal areas, and then in 1991 when 133,000 people died.
"Bangladesh is quite capable of managing the disasters now because there are cyclone shelters all along the coastal areas."