The already weakened wall of an alumina sludge reservoir is "very likely" to collapse, Hungarian Prime Minister Viktor Orban said on Saturday.

Orban made the remarks at a news conference in the western town of Ajka, saying cracks have appeared on the northern wall, which may lead to the collapse of the whole wall.

The toxic red sludge near the village of Devecser has caused seven deaths.

The sludge covered an area of 800 to 1,000 hectares, where the contaminated topsoil will have to be removed.