Workers are racing to build an emergency dam in western Hungary as cracks in a reservoir widened, threatening to unleash a second torrent of toxic sludge.
The village of Kolontar and nearby rivers are at risk, with officials saying 500,000 cubic metres of alumina waste could be unleashed.
Around double that spilled out last week after the reservoir partially collapsed.
Seven people have died while over a hundred are injured. Many homes have been deluged by the toxic red sludge which has also fouled a local branch of the Danube river.
Kolontar was evacuated on Saturday after the cracks appeared in the northern wall, threatening a second spill. The nearby town of Devecser remains on alert.
A by-product of alumina production, the thick, highly alkaline substance has a caustic effect on the skin. It contains heavy metals, such as lead, and is slightly radioactive. Inhaling its dust can cause lung cancer.
News agency MTI cited environment state secretary Zoltan Illes as saying a 25-metre-long crack in the weakened wall had widened slightly by Sunday morning and the wall of the damaged reservoir now looked beyond repair.
Illes said the northern wall of the reservoir could collapse "within one day or a week" and crews at the scene were scrambling to complete a new dam to protect Kolontar and the nearby town of Devecser, home to 5,400 people.
Prime Minister Viktor Orban has described the torrent of sludge as the worst ecological catastrophe Hungary has suffered.
The interior ministry said on its website that samples taken early on Sunday showed that alkalinity levels in smaller rivers affected by Monday's spill, and in the Danube, had returned to normal.