They say alkaline levels that killed all fish in one river were now greatly reduced, but were being monitored.

PM Victor Orban called the spill an "ecological tragedy". There are fears the mud, which burst out of a reservoir on Monday, could poison the Danube.

Countries downstream from Hungary, including Croatia, Serbia and Romania, are drawing up emergency plans.

So far, no dead fish have been spotted in the Danube itself.

Monday's accident at the Ajkai Timfoldgyaran alumina plant in Ajka in western Hungary killed four people and injured more than 100.

It also caused massive damage in nearby villages and towns, as well as a wide swathe of farmland.

Disaster official Tibor Dobson said all life in the Marcal, which feeds the Danube, had been "extinguished".

On Thursday Mr Orban visited the village of Kolontar, the worst-affected settlement, and said some areas would have to be abandoned.

"Hungary is strong enough to be able to combat the effects of such a catastrophe. But we're still open to any expertise which will help us combat the pollution effects," he added.

MAL Rt, the Hungarian company which owns the plant, said that, by EU standards, the sludge had not been considered hazardous.