The BBC's Mike Wooldridge in Dhaka: "Misery increases daily"

Turag and Buriganga rivers overflow embankments in Dhaka

The UN's Disaster Assistance chief in Bangladesh, Michael Elmquist, part of the team assessing flood relief needs, said if floodwaters remain for five more days, about 10m would be at risk and, should waters not recede in 20 days, 20m people may perish.

He said the disaster could only be alleviated if a huge relief operation is launched.

The warning came as the International Red Cross said it was fighting a losing battle in its efforts to get relief supplies and safe drinking water to stricken villages across the country.

Dhaka's embankment protection department pile up the sand bags

Monsoon-swollen rivers are continuing to rise around the capital city, Dhaka, where, according to field workers, the potential for disaster is enormous.

"The situation is now alarming in Dhaka," the state-owned Bangladesh Television said of the city, two-thirds of which is now under water with floods threatening parts of the international airport.

Officials in Bangladesh say Dhaka tops the danger list of cities facing complete devastation.

Continual efforts are being made there to prevent the collapse of an embankment protecting up to 500,000 homes on the eastern side of the city.

The Flood Forecasting and Warning Centre said the swollen Buriganga river is likely to rise even higher than its current level, already a metre above the danger level.

A truck stuck north of Dhaka on a flooded highway

Besides Buriganga, three other rivers encircling Dhaka are straining against their banks, prompting thousands of residents to help soldiers repairing protective dykes, especially wherever any seepage appears.

So far, the floods have killed more than 700 people and left more than 21m homeless, and it is still believed that the waters could rise further before they recede.

And as Dhaka's hospitals continue to fill up with patients suffering severe diarrhoea, foreign governments have issued health warnings to their nationals resident in the country.

Foreign relief pledged

The Bangladesh Prime Minister, Sheikh Hasina, has been defending her government against opposition criticism that the country was late in asking for aid and inadequately prepared for the disaster.

Pledges of foreign aid are arriving in response to Bangladesh's appeals for $879m for emergency relief and post-floods rehabilitation.