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Mud from a dam that burst at an iron ore mine in Brazil earlier this month, killing 12 people and polluting an important river is toxic, the United Nations' human rights agency said on Novemebr 25, 2015.

Meanwhile, 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.

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". They did not identify the studies that were the basis for the evidence or say who conducted them.

The owners of Samarco said they are taking every measure possible to provide emergency assistance to those affected by the dam burst and to reduce the social and economic impacts of the disaster.

Samarco said in a statement that both pre- and post-disaster tests show the mud released in the dam burst, made up mostly of water, iron oxides and silica or quartz, presents no danger to human health and does not contain water contaminants.

"They will not change chemical composition in water and will behave in the environment like normal soils in the catchment," BHP said. They declined to comment on UN's report.

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

BHP said the fish were most likely being choked to death by the sheer volume of fine sediment released by the dam burst clogging their gills, rather than the chemical composition of the sludge.

"Based on the evidence available, the concentration of sediment suspended in the water in the Rio Doce, as a result of the tailings being released, was high enough to starve the fish of oxygen," a BHP spokesman said.