BUDAPEST (AFP) – Hungary's toxic sludge spill, which has killed four people, reached the Danube river Thursday, threatening to contaminate the waterway's ecosystem, a water authority official told AFP.

Water alkalinity, a measure of river contamination, was already above normal in the major waterway, the official said.

Samples taken at the confluence of the Raab river and the Danube showed "alkalinity slightly above nornal, around 8.96 percent to 9.07 percent," against a normal tally of eight percent, the source said.

A wave of toxic mud was unleashed Monday from the reservoir of an aluminium plant at Ajka, 160 kilometres (100 miles) west of Budapest.

The red mud travelled down the Raab river and reached the Danube waters at around 0630 GMT at Gyor.

The industrial accident triggered by the collapse of walls at the factory reservoir on Monday has been described as an ecological disaster and is now threatening the entire ecosystem of the Danube, Europe's second longest river.