The Bangladesh authorities have closed down the airport in the north-eastern city of Sylhet after monsoon flood waters swamped its runway.
Many town and villages in Bangladesh are cut off
Forty-one of the country's 64 districts are now affected by the floods, and officials say 14 million people are either marooned or homeless.
Troops have been sent to help with relief and rescue operations.
Meanwhile, India's prime minister has set up a task force to deal with flooding in the badly-hit north-east.
Army called in
Bangladeshi officials say the overall flood situation in the country has worsened as fresh areas have been submerged by monsoon rains in the last 24 hours.
The conditions are being described as the worst floods in decades.
The international airport at the Bangladeshi town of Sylhet is knee-deep underwater, forcing the authorities to suspend all flights.
The airport, which is used by the national airliner Bangladesh Biman for flights to London and the private GMG Airlines for domestic flights, has been declared closed for an indefinite period.
But the BBC correspondent in Dhaka says that the worst affected district in the region is Sunamganj, where the entire town is now over a metre under water, with power supplies suspended for the last three days.
"The entire district is under water and there is no dry space in Sunamganj," the Relief and Disaster Management Minister, Chowdhury Kamal Ibne Yusuf, told the BBC.
The army has started relief operations in Sunamganj and other areas using helicopters and boats.
Railway communications have been suspended in many parts of the country, including the Dhaka-Sylhet route.
The Relief Ministry says more than 60 people have been killed, and that more than a million acres of crops have been destroyed.
The Flood Warning Centre says the River Buriganga on the southern side of Dhaka continues to rise fast, threatening the capital.
The government says it has enough food stocks to continue the relief operation for at least three months.
Meanwhile, the flood situation in India's north-east continues to cause concern, with the prime minister - on a tour of the area - announcing the formation of a task force to deal with the problem.
"The flood situation is very grim," he said.
More than 100 people have been killed and 10 million made homeless by floods in the states of Assam and Arunachal Pradesh, where rains continued for a sixth day.
Inadequate relief effort
At Dibrugarh, Assam's second largest town, the Brahmaputra was flowing more than two metres above the danger mark.
The BBC correspondent in Calcutta says that the embankment which protects the town from the river has developed large leaks, threatening Mohanbari airport and the army cantonment.
The administration has asked the town's population of one and a half million people to move to safer areas.
The word 'monsoon' comes from the Arabic for 'season'
Describes seasonal reversals of wind direction
From April heat builds over South Asia, creating low pressure areas
Brings moisture-rich south-west winds in from the ocean
Our correspondent says a similar picture is emerging in other parts of Assam and Arunachal Pradesh, with bridges, embankments and other facilities collapsing as more and more villages go under water.
With landslides on national highways, the states of Nagaland, Manipur, Mizoram and Tripura have been cut off from the rest of the country.
Officials say floods in the eastern state of Bihar are also worsening, with waters from the Ganado, Baghdadi and Koki rivers surging sharply and inundating fresh areas.
Around 100 people have been killed in the state and millions marooned.
The state government says the army has air-dropped about 226 tonnes of food with the help of helicopters.
There are also reports of flood victims looting emergency supplies in Bihar, complaining that the relief effort is inadequate.