In the northern Indian state of Bihar, thousands more people are being evacuated to higher grounds to escape the rising flood waters.
Angry villagers in the worst-affected district, Madhepura, said they had no idea where to take shelter.
At least 55 people have died and 2.5m are displaced after the Kosi river breached its banks, changing course.
The flooding began 11 days ago in neighbouring Nepal leaving tens of thousands of people homeless there.
On Thursday Indian PM Manmohan Singh visited the affected areas in Bihar. He said the flood was a "national calamity".
The prime minister has announced an aid package worth $230m.
But aid agencies say many of the victims are being moved to temporary shelters which lack basic amenities and are at risk from disease.
'Under water'
Mohanlal Sharma of the BBC's Hindi service in Madhepura says the road between Madhepura and Purnea districts is under 10 feet of water.
"Along the route, I saw thousands of villagers running towards the town. Some were on foot, some on their bicycles. Some were taking their cattle. Villagers have loaded their entire life's acquisitions on ox-carts and trying to escape the waters."
Our correspondent says people have taken shelter at the railway stations and higher stretches along some of the roads.
Many angry villagers complained they had received no food or aid and did not know where they could take shelter.
"Every where you look, there is water. Houses are submerged, shops are under water and people can be seen wading through water."
Meanwhile, officials say a massive rescue and relief operation is underway.
At least 124,000 people have been evacuated to safer grounds.
Officials said 725 villages were badly affected by the floods and 106 relief camps had been set up in the area.
Army and air force helicopters continued to provide aid to the flood-ravaged parts and 600 boats were helping with the relief and rescue work.
Prime Minister Manmohan Singh and the president of the governing Congress Party, Sonia Gandhi, flew over the flood-affected area on Thursday by helicopter to get a first-hand view of the devastation.
The floods have washed away roads and railway tracks, and water and electricity supplies have been affected in many areas.
"This situation is beyond comprehension," Bihar resident, Arshad Khaqani, told the BBC News website.
A report released by Unicef says there are fears of infectious diseases as hundreds of thousands of homeless crowd into relief camps.
Nepal havoc
The Kosi river flows from Nepal where it is called the Saptakoshi river. On 18 August a dam on the Saptakoshi burst, triggering the subsequent flooding in Bihar.
Officials in Nepal say hundreds of people there have been hit by illnesses such as diarrhoea and pneumonia and an estimated 50,000 are homeless.
They say nearly 1,000 houses have been completely destroyed. Power supplies and transport have been severely affected.
The costs to the economy are now estimated at one billion Nepalese rupees ($14.25m).
Meanwhile, in India's Uttar Pradesh state, which borders Bihar and Nepal, the health authorities say at least 400 people have died from water-borne diseases during the current monsoon season.
Officials say 181 of the deaths have occurred in the eastern districts from encephalitis carried by mosquitoes breeding on the water.
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Many people say they have no idea where to take shelter
BBC