Hungary has declared a state of emergency in three counties after a flood of toxic red sludge from a chemical plant engulfed several towns.
One official called it "an ecological disaster" that may threaten the Danube and other key rivers.
The toll rose to four dead, six missing and at least 120 people injured after a reservoir failed at the Ajkai Timfoldgyar plant in Ajka, a town 100 miles south-west of Budapest, the capital.
Several hundred tons of plaster were being poured into the Marcal River to bind the toxic sludge and prevent it from flowing on.
So far, about 35.3 million cubic feet of sludge has leaked from the reservoir.
Environmental Affairs State Secretary Zoltan Illes called the flood an "ecological catastrophe" and said the sludge could reach the Raba and Danube rivers. He suspended activity at the plant and ordered the company to repair the damaged reservoir.
Disaster workers said 390 residents had to be temporarily relocated and 110 were rescued from the flooded towns, including Kolontal, Devecser and Somlovasarhely. Firefighters and soldiers swept through the region carrying out cleanup tasks with bulldozers.
The sludge, a waste product in aluminium production, contains heavy metals and is toxic if ingested. Many of the injured sustained burns as the sludge seeped through their clothes, and two faced life-threatening conditions. Two women, a young man and a three-year-old child were killed in the flooding.
The injured were being monitored because the chemical burns caused by the sludge could take days to emerge and what may seem like superficial injuries could later cause damage to deeper tissue.
Local environmentalists say they have tried to call the government's attention to the risks of red sludge for years, pointing to a 2003 report in which they estimated the waste at 30 million tons.