More than half of the 64 districts of Bangladesh are now affected by flooding, with officials warning that the overall situation has deteriorated.
Much of Bangladesh is waist deep in water
In conditions described as the worst floods in decades, the three largest rivers in the country are all rising as monsoon rains continue.
The north-east of India is also still badly hit, with one town in Assam state facing complete submergence.
It is estimated that about 20 million people in South Asia are marooned.
Bangladeshi officials say that 12 million have been hit by flooding in 33 districts of the country, and that the capital Dhaka remains under threat.
At least 57 people have died, officials say, but the BBC correspondent in Dhaka says that newspaper reports suggest the total is at least double that.
The Flood Warning Centre in Dhaka say that the Brahmaputra, Ganges and Maghna rivers continue to rise, leaving people either stranded or forced to seek shelter on higher ground.
The Bangladeshi government says it is monitoring the situation, and is continuing to send relief materials - including food and medicines - to the inundated areas.
Flooding in 1998 killed more than 3,500 people and submerged two-thirds of the country.
"We all are in knee-to-waist deep water," a police spokesman in the town of Manikganj - west of Dhaka - told the Reuters news agency.
"The situation is very bad and it is likely to worsen further if heavy downpours continue," he said.
Expecting the worst
Meanwhile, the second largest town in the north-east Indian state of Assam is facing complete submergence as weeks of torrential rain continue to bring misery to millions in South Asia.
Officials say that the River Brahmaputra has risen to an all time high, threatening the 1.5 million residents of Dibrugarh.
Other towns in the state are already reported to be at least two metres under water.
The BBC correspondent in Calcutta says that residents of Dibrugarh are expecting the worst.
Our correspondent says that the fear is that if the embankment around the town gives way, it will not be easy to rescue its inhabitants.
Many villages and towns in Assam are also under water
Officials say that around 10 million people in the state are affected by the second wave of flooding in Assam this year, with road and rail communications almost non-existent.
But in India's eastern state of Bihar, officials say that flood waters are slowly receding, even though more than half of the town of Khagaria remains submerged.
About 130 people have died in Bihar since the flooding began.
Floods and landslides are common in South Asia during the monsoon season, when annual rains combine with melting snow from the Himalayas.
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