Hungary raced against time yesterday to erect a dam around a ruptured reservoir and divert a new wave of toxic sludge that threatens to overwhelm already devastated villages.

As hundreds of volunteers joined engineers rushing to erect the 600-metre dam, a top official said it was only a matter of days before the reservoir housing a chemical residue would begin to crumble.

“In two or three days there is going to be rain and we are trying to speed things up so that we can finish off the dam before the rain comes,” Zoltan Illes, the state secretary for environmental protection, told reporters at the dam construction site.

“Once the rain is here, the remaining sludge will be washed out and the dam’s northern section is going to break away. This is imminent. Once the wall breaks down, the sludge will start flowing again.”

Repair work also continued on the reservoir itself where cracks have been detected, raising fears that what is already Hungary’s worst environmental disaster could soon get even worse.

At least seven people were killed when the red-coloured sludge first began seeping from the reservoir next to an alumina plant before then cascading into nearby villages and tributaries of the Danube.

Kolontar was the village worst hit by the disaster and its entire population of nearly 1,000 people have been forced to evacuate while the threat of a new spill remains.

Many have been put up in a sports centre in the nearby town of Ajka although others have moved in with relatives.

Tibor Dobson, the official in charge of the disaster relief effort, said around 800 volunteers were helping 900 police, disaster relief workers and fire-fighters yesterday.

The country’s public health service said all relief workers must wear breathing masks and protective glasses, as drying sludge was starting to turn to cause dust.