BUDAPEST - Hungary's prime minister said it was likely the weakened northern wall of an alumina sludge reservoir will collapse, Prime Minister Viktor Orban said on Saturday.

"Last night the interior minister informed us that cracks have appeared on the northern wall of the reservoir, whose corner collapsed, which make it likely that the entire wall will collapse," Orban told a news conference in the western town of Ajka.

He said about 500,000 cubic metres of red sludge could spill out of the reservoir, but this substance was thicker than the roughly 1 million cubic metres which tore through the villages nearby this week, killing seven people and injuring over 100.