Heavy rainfall and onrush of water from the upstream submerged low-lying areas in Rangamati, Khagrachhari, Sylhet, Sunamganj and Nilphamari districts.
Various parts of Barkol and Bilaichhari upazilas in Rangamati went under water following torrential rains and onrush of water from hills.
Besides, water in the Kaptai Lake was flowing above the danger level on Wednesday, reports UNB.
Sources at Kaptai Water and Electricity Centre said the authorities concerned have opened eight out of 16 gates of the spillway of the dam to release 4500 cusec water to the Karnaphuli River to avoid the danger and handle the excessive pressure of water.
Meanwhile, the road communications between Chattogram and Rangamati remained suspended since Tuesday morning due to landslides at different points on the road.
However, communications on internal routes in Rangamati remained normal.
In Khagrachhari, different parts of Dighinala upazila, which had experienced serious flooding barely two weeks ago, were flooded again on Wednesday due to torrential rains for the last two days.
Locals said Dighinala-Langdu road went under water, disrupting communication.
Farmers fear a huge loss of crops due to the flood.
In Sunamganj, thousands of people have got marooned in Dowarabazar, Shalla, Tahirpur, Dharamapasha upazilas due to flood caused by the rains.
The Surma River was flowing 80 centimetres above the danger level at Sholosahar point.
Besides, low-lying areas -- Sahebbari, Nabinagar and Natunbazar -- of the town went under water and many educational institutions were shut as those got swamped by rainwater.
The local Water Development Board has recorded 120mm of rainfall till Wednesday morning.
District Relief and Rehabilitation officer Faridul Haque said that every upazila has been allocated 10 tonnes of rice and Tk 50,000.
In Sylhet, over 100,000 people have got marooned as flood caused by heavy rains inundated low-lying areas in Goainghat upazila of the district.
The Piyain and Sari rivers are flowing 46 metres above the danger level.
In Nilphamari, different villages in Dimla and Jaldhaka upazilas were flooded due to the rise of water level in the Teesta River, said local public representatives.
The Teesta was flowing 5 centimetres below the danger level at Teesta Barrage point, said Dimla Water Development Board official Nurul Islam adding that 44 water gate of Teesta Barrage were kept open.
Heavy to very heavy rainfall with gusty wind is likely to occur at places over Rangpur, Mymensingh, Sylhet and Chattogram divisions due to active monsoon during the next 24 hours commencing 6 pm on Wednesday.
Due to very heavy rainfall landslide may occur at places over the hilly region of Sylhet and Chattogram divisions, said a Met Office bulletin.
