Hungary says it will cost tens of millions of dollars and take at least a year to clean up the damage caused by a spill of toxic, red industrial sludge.
Emergency workers are trying to stop the spill, from an alumina plant, from flowing into major waterways, including the River Danube
A state of emergency has been declared in three western counties after the chemical waste burst from a reservoir.
Four people are known to have died, and 120 were injured. Six more are missing.
At least seven villages and towns are affected including Devecser, where the torrent was 2m (6.5ft) deep.
The flood swept cars from roads and damaged bridges and houses, forcing the evacuation of hundreds of residents.
The sludge - a mixture of water and mining waste containing heavy metals - is considered hazardous, according to Hungary's National Directorate General for Disaster Management (NDGDM).
While the cause of the deaths has not yet been officially established, the victims are thought to have drowned.
Some 600,000-700,000 cubic metres (21m-24m cubic feet) of sludge escaped from the plant, 160km (100 miles) from the capital, Budapest, affecting an area of 40 sq km (15.4 sq miles).
Environment Minister Zoltan Illes told the BBC the clean-up would take at least one year and probably require technical and financial assistance from the European Union.
He described the spill as Hungary's worst chemical accident.
"The area is very big, very heavy contamination, lots of human resources are needed, definitely machinery is needed," he said.
Mr Illes said a layer of soil 2cm deep (0.8 inches) would need to be removed from the whole of the contaminated region.
With 7,000 people affected directly by the disaster, a state of emergency was declared in the county of Veszprem where the spill occurred, and Gyor-Moson-Sopron and Vas, where the sludge appeared to be heading.
At least 390 residents have been relocated and 110 rescued from flooded areas, the NDGDM said.
Nearly 500 police officers and soldiers, including six emergency detection teams, have been deployed. Plaster has been poured into the Marcal river in a bid to bind the sludge and stop further flooding.
An alert has been declared for the Marcal and Torna rivers, and Mr Illes said workers were "desperately" trying to stop contamination of the Raba and Danube rivers.
The BBC's Nick Thorpe is in the village of Kolontar, the first and worst affected of the settlements just downstream from the burst containment pond.
The dark red colour pervades everything, he says: the streets, the sides of the damaged houses and the forbidden zone beyond.
The army hopes to install a pontoon bridge to reach the part of the village cut off since the old bridge was swept away by the mud on Monday.
That is where the damage is biggest and where several people lost their lives, our correspondent says.
Dr Attila Nyikos, of the NDGDM, told the BBC News website that a police investigation had been opened and tests were still being carried out to determine the environmental impact of the leak.
The sludge escaped from a reservoir at the Ajkai Timfoldgyar plant in the town of Ajka. Police say they have confiscated documents from the company's headquarters.
The plant makes alumina, a synthetically produced aluminium oxide. It is a white or nearly colourless crystalline substance that is used as a starting material for the smelting of aluminium metal.
Weeks of heavy rain are likely to have played a role in the accident, the BBC's Nick Thorpe reports.
MAL Rt, the Hungarian company which owns the plant, earlier said that by EU standards the sludge had not been considered hazardous.
There had been no sign of the impending disaster and the last examination of the reservoir pond on Monday had shown nothing untoward, it added.
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