The BBC's David Chazan reports from Dhaka

Troops, police, civil engineers and volunteers are battling to save the vital barrier from bursting under the surging waters.

With fears of further flooding in the capital in the next few days, officials have drawn up evacuation plans.

Floods have brought disease

Two-thirds of the country is inundated, and more than half the capital is already knee to waist deep in filthy water.

More than 800 people have died and at least 21 million are homeless.

Army personnel and civilians worked through the night to try and save the Dhaka-Narayanganj-Demra (DND) dike from flood waters.

"We have been trying to save the DND dike at all costs," an official of the Bangladesh Water Development Board (BWDB) said.

"So far it's holding...but it seems more vulnerable today. I don't know what will happen."

A senior engineer with the board said if the embankment collapsed, up to 2.5 meters of water would cover the area.

But it would take at least eight hours to reach that height, giving people time to evacuate.

The embankment also serves as a highway linking Dhaka to the the towns of Demra and Narayanganj.

Flood barriers are seeping water

Overnight efforts helped to plug hundreds of seeping rifts along the 17 kilometre structure, but residents reported many had reopened on Thursday morning.

Water Resources Minister Abdur Razzak minister said authorities were considering emergency evacuation in case the embankment gave in.

Flood forecasters in Dhaka said a 2.5 km stretch of the embankment had been weakened by the overflowing of the Buriganga and Sitalakhiya rivers.

Both rivers are likely to swell further under the impact of gushing waters from their upper reaches.

Disease spreading

The River Ganges has risen to its highest level on record.

Diseases like diarrhoea are spreading because many sources of clean drinking water have been covered by dirty flood waters.

The price of rice has risen, and food shortages are feared as the worst floods for a hundred years have washed away $300m worth of crops.

Volunteers held deliver aid

The Bangladeshi government says enough aid is reaching those in need. But relief agencies say there are enormous difficulties in getting food and medicines to stranded communities.

The Red Cross says some boats refuse to carry emergency supplies because river currents have become too strong.

The United Nations has appealed for more than $200m in emergency aid. Donors have so far pledged about half of that amount.

Roads between Dhaka and many districts including the main port, Chittagong, have been cut for weeks.