Casualties from a collapsed dam at a Brazilian mine owned by Vale and BHP Billiton mounted on Friday after rescue teams worked through the night to find the dozens missing in mudslides that devastated a nearby village.

A spokesman for firefighters in the rescue confirmed 30 injuries and at least two deaths, but said the count was likely to rise as the search advanced slowly after mudslides knocked out roads and cellular towers.

"In reality there are a lot more, but we can't confirm any more than that. We don't even know that we'll find everybody," said firefighter Adão Severino Junior in Mariana. He said the local hospital was "saturated" and victims arriving by helicopter were being sent to nearby Ouro Preto.

BHP Billiton Chief Executive Officer Andrew Mackenzie said at a news conference that full assessment of casualties and damage was hampered by nightfall that came a few hours after the incident on Thursday.

Vale directed questions to officials at their Samarco joint venture, which said in a statement it had not yet determined why the dam burst or the extent of the disaster.

The dam was holding tailings, a mining waste product of metal filings, water and occasionally chemicals. It was located near the Gualaxo do Norte river, adding to fears of potential water contamination.

The Germano mine produces 30 million tons of iron ore pellets per year. Samarco produces just under 10 percent of Brazil's iron ore output. Iron ore is transported down a slurry pipe from Germano to Espirito Santo, where it is turned into pellets and shipped to customers including the Libyan Iron and Steel Co (Lisco), one of North Africa's biggest steelmakers.