The toxic red sludge overflow from a Hungarian factory which destroyed three villages today reached the Danube, Europe's second-longest river.
As well as providing much of the country's drinking water, the river flows through Croatia, Serbia, Romania, Bulgaria, Ukraine and Moldova before emptying into the Black Sea.
The EU had feared an environmental catastrophe if the sludge, a waste product of making aluminium, contaminated the 1,775-mile long Danube.
It overflowed a reservoir on Monday and emptied into local streams that feed waterways connected to the Danube.
Those in Kolontar, 45 miles from the Danube and the closest town to the spill site, were swollen ochre red and villagers said they were empty of fish.
The sludge reached the western branch of the Danube early today. The Hungarian rescue agency did not discuss its possible toxic metal content but said its acidity had been reduced to the point where it was unlikely to cause further damage to the environment.
It said the pH content, which had hit 13, was now under 10. Normal ph levels for surface water range from 6.5 to 8.5.
At the Croatian village of Batina, the first site after the Danube leaves Hungary, experts were taking water samples today which they will repeat.
In Romania, the water was reported safe, with testing being carried out every three hours.
Hungarian Prime Minister Viktor Orban visited the three villages coated by the red sludge today and declared the worst-hit area a write-off, saying he saw "no sense" in rebuilding in the same location.
Residents said the disaster had destroyed the whole community of 800 by making their land valueless.
Angry villagers gathered outside the mayor's office. "The whole settlement should be bulldozed into the ground," bellowed Janos Potza. "There's no point for anyone to go back home."
"Those who can, will move out of Kolontar. From now on, this is a dead town," fumed Beata Gasko Monek.
It is still not known why part of the reservoir collapsed. Authorities have ordered a criminal inquiry into the accident, which killed at least four people, injured 120 and left three people missing amid an estimated torrent of 35 million cubic feet of toxic waste.
A spokeswoman for the National Police said investigators would look into whether a work mistake was a factor.
The huge reservoir, more than 1,000 feet long and 1,500 feet wide, was no longer leaking and a triple-tiered protective wall was being built around its damaged section. Guards have been posted at the breach to give an early warning in case of any new emergency.
The sludge spill is "one of the top three environmental disasters in Europe in the last 20 or 30 years," said Greenpeace.
The International Commission for the Protection of the Danube, which manages the river and its tributaries, said the spill could trigger long-term damaging effects for both wildlife and humans.
Philip Weller, the group's executive secretary, said factories and towns along the Danube may have to close their water intake systems. He said large fish in the Danube could ingest any heavy metals carried downstream, potentially endangering people who eat them.
Red sludge is a by-product of the refining of bauxite into alumina, the basic material for manufacturing aluminium. Treated sludge is often stored in ponds where the water eventually evaporates, leaving behind a dried red clay-like soil.
Hungarian company officials have insisted the sludge is not considered hazardous waste according to EU standards. The company has also rejected criticism that it should have taken more precautions at the reservoir.
Alumina plants are scattered around the world, with the 12 largest concentrated in Australia, Brazil and China. The plant in Hungary ranks 53rd in the world in production, according to industry statistics.