The flood situation in eastern districts turns worst as 3.0 million people have remained stranded for days, forcing army deployment in places for rescue operation.
The rescuers struggled to reach some worst-hit unions in Sylhet and Sunamganj with the onrush of upstream water amid Friday's torrential rain as millions wait to be taken to safety.
This is the second wave of flash flood of such magnitude experienced by the districts just in a month.
Meanwhile, parts of Moulvibazar, Netrakona, Sherpur, Lalmonirhat, Nilphamari, Sirajganj and Kurigram also went under water as rivers of two major basins-the Ganges and the Brahmaputra were in floods.
Humanitarian assistance should reach thousands of people immediately as the entire Sylhet and Sunamganj districts went under water amid record rainfall in 122 years in the neighbouring Indian states of Assam and Meghalaya, said experts.
Inadequate preparations by the disaster management authorities have made the situation critical as rescue work became toughest in many places amid vessel shortages and cuts in road communications, they added.
The whole of Sunamganj and most parts of Sylhet districts have been physically disconnected amid the worst flood the region has ever experienced.
Bangladesh Water Development Board Sylhet executive engineer Asif Ahmed said water was flowing 1.28 metres above danger level in the Surma flooding the entire district except Fenchuganj and Balaganj.
He said virtually the total population (nearly 3.9 million) of Sylhet is now under water.
The Cherapunji in Meghalaya of India witnessed a 122-year record rainfall of 972 milimitre just in a day while the Meghalaya and Assam states of India witnessed a 2500mm rainfall in last three days.
He said the condition might deteriorate further as the rain might continue for three to four days.
The Kushiara might also swell above the danger mark within a few days thus could make the condition critical further, added.
According to the Met office, Sylhet has also witnessed maximum 268mm of rainfall in the last 24 hours.
Sunamganj experienced maximum 375mm rainfall during the period, said the Met office.
Floods have already disconnected Sunamganj from the country as water was overflowing roads with huge current and prompting suspension of the national power grid operation in the region since Thursday night.
Stranded flood victims told their relatives over phone of being clumped atop roofs as floodwaters passed just under the eaves.
The situation turned worst beyond imagination by Friday evening as many flood victims of Sunamganj were even unreachable through mobile amid poor network or handed ran out of charge for no electricity.
In Sylhet, army along with power development officials dumped geo-bags to hold back upstream water from entering a power plant and transmission substation.
The authorities have confirmed that more than 1.0-million electricity consumers have been without power in Sylhet, Sunamganj and Netrakona.
Sylhet divisional commissioner Muhammad Mosharrof Hossain told the FE that more than 3.0-million people need to be rescued immediately in the region.
The administration is giving priority to save lives first, he said.
Mr Hossain said, "If rain continues for the next few days, we have to rescue above 5.0 million people."
State minister for disaster management and relief Enamur Rahman said this year's flood is the worst in seven decades.
The situation is so critical that they could not distribute relief on Friday, he added.
Osmani International Airport also cancelled all flights for the next three days as water flooded its runway.
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