The owner of a mining waste reservoir that burst in Brazil last month, killing 10 people and causing a huge environmental disaster, says two more are at risk of breaking.

Crews are placing rocks along those two reservoirs in southeastern Minas Gerais state to shore them up. At one of them, the work will take 45 days and at the other 90, said Germano Lopes, managing director of the mining company Samarco.

"We are taking the necessary urgent measures," he told a news conference, a recording of which was obtained by AFP on Wednesday.

The November 5 disaster unleashed a torrent of yellowish muck that contaminated drinking water supplies and mostly destroyed the village of Bento Rodrigues. Besides the 10 deaths, 15 people are still missing.

The spill contaminated some 500 kilometers (310 miles) of the Doce River in Minas Gerais and the neighboring state of Espirito Santo, destroying crops and killing fish, turtles and other animals.

Samarco initially said that two reservoirs burst. But Lopes told the press conference it was actually just one that gave way, and a second overflowed.

Now, the one that overflowed and another nearby are at risk of breaking, Lopes said.

Samarco is a joint venture of Australia's BHP Billiton, the world's largest mining firm, and Brazil's Vale, the biggest iron ore miner.

On Tuesday authorities declared a state of emergency in more than 200 towns affected by the spill.

The 180-day state of emergency gives victims access to disaster relief funds, speeds the deployment of emergency workers and cuts through local governments' red tape for infrastructure projects and purchases of medicine.

The muck flowing in the river for the past 13 days should reach the Atlantic Ocean Friday, the state-run Brazilian Institute of Environment and Renewable Natural Resources said.

Environment Minister Izabella Teixera has said it will take at least a decade for the river to recover fully.

But environmental experts have said the environmental fallout of the disaster could drag on for a century.

Samarco has pledged to pay $260 million in damages as a preliminary commitment toward clean-up and compensation.

The courts have blocked an additional $78 million in Samarco funds in banks to pay out damages. And the government has slapped the firm with $67 million in fines and warned that more fines are to come.

The final clean-up cost could top $1 billion, according to Deutsche Bank.