The walls of a red sludge reservoir in Hungary were "very likely" going to collapse, the prime minister said Saturday as the government rushed to prevent another deluge like the one earlier this week that killed at least seven people.

Prime Minister Viktor Orban described the situation as "dramatic" and said the town of Kolontar, which sits next to the reservoir and was badly damaged in Monday's flood, was evacuated as a precaution because engineers have determined that the developing cracks could cause a whole side of the enormous container to collapse.

"Cracks have appeared on the northern wall of the reservoir which makes it very likely that the whole wall will collapse," Orban told reporters gathered at a fire station in Ajka, a city near Kolontar where many of its resident have been evacuated. "The wall is in very bad shape."

"We have started to build dams in the direction of the populated areas to slow the flow of the material in case of a new incident," Orban said.

The prime minister said experts had estimated that _ depending how dense the sludge was _ an estimated 500,000 cubic meters more of red sludge could escape from the reservoir if the wall collapsed, but said exact figures were hard to calculate. Orban said that amount would be about half the scale of the initial sludge flood.

"We have no exact information about the nature of the material because a catastrophe like this has never happened before anywhere in the world," Orban said. "We have only assumptions about how far and with how much force the material can come out of the storage container."

Orban added that the incident could have been avoided and that "there was no information diminishing the responsibility" of human error as a causing factor.

"Hungary has never experienced any tragedy like that and we are all astonished," Orban said. "Human errors and mistakes must exist ... and the (legal) consequences will be very serious."

In addition to the fatalities, more than 120 were injured when a corner of the metals plant reservoir gave way and up to 700,000 cubic meters (184 million gallons) of toxic waste flooded several towns in western Hungary. The amount was not much less in under an hour than the 200 million gallons (757 million liters) the blown-out BP oil well gushed into the Gulf of Mexico over several months.

But the concentration of toxic heavy metals where Hungary's red sludge spill entered the Danube has dropped to the level allowed in drinking water, authorities said, easing fears that Europe's second longest river would be significantly polluted.

The red sludge devastated creeks and rivers near the spill site and entered the Danube on Thursday, moving downstream toward Croatia, Serbia and Romania. Monitors were taking samples every few hours to measure damage from the spill but the sheer volume of water in the mighty Danube appeared to be blunting the red sludge's immediate impact.

Test results released by Hungary's disaster agency show the pH level of the water where the slurry entered the Danube was under 9 _ well below the 13.5 measured earlier in local waterways near the site of the catastrophe. That is diluted enough to prevent any biological damage, Interior Minister Sandor Pinter said.

Despite the apparent good news, the risk of pervasive and lasting environmental damage remained at the site of the spill, with Greenpeace presenting laboratory tests that it said showed high concentrations of heavy metals in the sludge.

MAL Rt., the Hungarian Aluminum Production and Trade Company that owns the Ajkai Timfoldgyar plant where the spill occurred has rejected criticism it should have taken more precautions at the reservoir.

Hungarian police have confiscated documents from the company, and the National Investigation Office was looking into whether on-the-job carelessness was a factor in the disaster.

Authorities began questioning people in the case and were looking for witnesses who could provide information about the reservoir's operations and maintenance work.

There are red sludge storage sites at several other locations in western Hungary, holding at least 30 million cubic meters (1 billion cubic feet) of the material.