Mining giants responsible for a dam spill in Brazil earlier this month rejected Thursday accusations by the United Nations that "20,000 Olympic swimming pools of toxic mud" spewed into a river.

The November 5 collapse of a waste-water dam at the Samarco iron ore mine owned by the world's biggest mining corporation BHP Billiton and its Brazilian partner Vale triggered a flood of muddy water that killed at least 13 and left 11 people missing, according to the latest toll.

The question now is what's in the reddish sludge gushing down the River Doce from the mine and out into the Atlantic.

Two UN environment experts issued a statement late Wednesday accusing the government and Samarco's owners of failing to confront -- or even acknowledge -- a toxic disaster.

Fifty million tons of waste, including "heavy metals and other toxic chemicals," were released into the area, the experts said.

"The scale of the environmental damage is the equivalent of 20,000 Olympic swimming pools of toxic mud waste contaminating the soil, rivers and water system of an area covering over 850 kilometers," the UN special rapporteur on human rights and the environment, John Knox, said.

"It is not acceptable that it has taken three weeks for information about the toxic risks of the mining disaster to surface," Knox and Baskut Tuncak, the special rapporteur on human rights and hazardous substances and wastes, said in a statement.

"The steps taken by the Brazilian government, Vale and BHP Billiton to prevent harm were clearly insufficient. The government and companies should be doing everything within their power to prevent further harm, including exposure to heavy metals and other toxic chemicals," they said.

- Danger free -

But Australia's BHP Billiton said the water and debris posed no danger.

"The tailings that entered the Rio Doce were comprised of clay and silt material from the washing and processing of earth containing iron ore, which is naturally abundant in the region," the corporation said in a statement.

"Based on available data, the tailings are chemically stable. They will not change chemical composition in water and will behave in the environment like normal soils in the catchment."

Brazil's Geological Service, which depends on the mining and energy ministry, appeared to support this Thursday, saying that new studies show an increase in heavy metals in the waste sediment, but "no indications that the mud is toxic."

The Brazilian navy announced that a research ship had arrived in the area and would spend the next four days collecting samples for environmental analysis.

"We will work every day around the clock to find solutions as quickly as possible," the ship's captain, Aluizio Maciel de Oliveira Junior, said.

According to Knox, the scale of the problem is already tragically clear.

The River Doce watershed "is now considered by scientists to be dead and the toxic sludge is slowly working its way downstream towards the Abrolhos National Marine Park where it threatens protected forest and habitat," he said.

"Sadly the mud has already entered the sea at Regencia beach, a sanctuary for endangered turtles and a rich source of nutrients that the local fishing community relies upon."