The statement contradicts claims by Samarco, the mine operator at the site of the rupture, and Samarco's co-owner, BHP Billiton, that the water and mineral waste contained by the dam are not toxic.

Citing 'new evidence', the UN's Office of the High Commissioner for Human Rights said in a statement the residue "contained high levels of toxic heavy metals and other toxic chemicals".

The owners of Samarco, Brazil's Vale SA and Australia's BHP, said they are taking every measure possible to provide emergency assistance to those affected by the dam break and to reduce the social and economic impacts of the disaster.

While iron and manganese levels in the mud are above normal, Samarco said, they are below dangerous levels.

The mud has killed thousands of fish as it flows through the Rio Doce, the river which connects the mineral-rich state of Minas Gerais with Espirito Santo on the Atlantic coast.

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