Hungarian authorities say a second flood of toxic sludge from a chemicals plant is likely after new cracks appeared in a dyke.
The village of Kolontar, which is close to the reservoir that burst in western Hungary on Monday, was evacuated at dawn.
Seven people have been killed and rivers have been poisoned in the country's worst ecological disaster.
Security forces have also warned thousands of residents in the nearby village of Devecser to be ready to be moved if necessary, officials said.
‘The reservoir is so damaged that it is likely that it will give way for a second time,’ Prime Minister Viktor Orban said.
National news agency MTI reported that the dam of the damaged part of the reservoir has further weakened and disaster crews decided to evacuate the village.
Earlier, it had been reported that officials said pollution levels from the red sludge spill had subsided in the Danube and there was no risk of a biological or environmental catastrophe in the river.
Interior Minister Sandor Pinter told a news conference the spill had not affected the drinking water supply so far and government spokeswoman Anna Nagy said the food chain was safe.
Hungary declared a state of emergency in three counties on Tuesday after a torrent of toxic sludge from an alumina plant tore through three villages 160km west of Budapest, killing seven people and leaving dozens with vicious burns.
A photograph circulated by environmental group WWF suggested the reservoir may have been leaking as far back as June. Officials at the Ajkai Timfoldgyar plant had no comment.
‘Let's not even consider the pollution that got into the Danube as real pollution now,’ Pinter said. ‘It will not be of an extent which would cause biological or environmental damage.’
Mr Dobson said pH levels were around 8, a ‘normal’ level, in the Danube, down from around 9 when the sludge reached the river on Thursday.
‘We have not experienced any damage on the main Danube so far,’ Dobson told Reuters. Crews have sought to dilute the alkaline content of the spill.
The cause of the accident remains unknown. But Gusztav Winkler, a professor at the Budapest Technical University who was part of a team that examined the area's soil 30 years ago, said a bad choice of location may have played a role.
‘Two entirely different soils meet here, one a sediment type and one of clay,’ he told Reuters. ‘When exposed to water, they expand, shift, to a different extent. They move.’
He said the collapsed corner of the reservoir was precisely where the clay and the sediment meet in the soil, which may have helped to create tension zones inside the dam's wall.
‘It's a rigid structure,’ he said. ‘You push it, it breaks.’
While there is a good chance the spill's impact on the Danube will be limited, western Hungarian villages that bore the brunt of the sludge torrent could suffer in the longer term, environmental group Greenpeace said.
Test samples from the sludge showed that government health and science agencies had underestimated the ecological dangers unleashed, Greenpeace Hungary Director Zsolt Szegfalvi said.
Arsenic, mercury and chromium levels were especially high at Kolontar, he said, rejecting earlier claims by the National Academy of Sciences in which it said the sludge contained no harmful levels of heavy metals.
‘This contamination poses a long term risk to both the water base and the ecosystem,’ a Greenpeace statement said.
More than 150 people were injured in the disaster, mainly burns and eye ailments from the caustic and corrosive sludge.
All waterlife died in the smaller Marcal River, first struck by the spill. There were also reports of sporadic fish death on Thursday in the Raba and Mosoni-Danube rivers. There were no reports of major damage to the main branch of the Danube.
Downstream from the disaster site, the Danube flows through or skirts Croatian, Serbian, Bulgarian, Romanian, Moldovan and Ukrainian territory en route to the Black Sea.
