Brazilian mining company Samarco says two dams it uses to hold waste water from iron production are damaged and at risk of collapsing.
One of the company’s reservoirs burst earlier this month, flooding dozens of homes in the south-eastern state of Minas Gerais.
Eleven people were killed and 12 are missing presumed dead.
Emergency work to try to avoid another breach will begin immediately and will last up to 90 days, the company said.
The company initially said that two of its dams – Fundao and Germano – had burst on 5 November.
But it has now clarified that only the Fundao reservoir collapsed.
Germano and another nearby dam, Santarem, are still standing but are at risk, said Samarco’s Infrastructure Director Kleber Terra.
Samarco is owned by by mining giants Vale, from Brazil, and Anglo-Australian company BHP Billiton.
It agreed on Monday to pay the Brazilian government 1bn reais (£170m; $260m) compensation.
The money will be used to cover the initial clean-up and to offer some compensation to the victims and their families.
But the amount agreed is relatively small, prosecutors said.
It will be considered “only as a first instalment” to be paid by the company, state prosecutor Carlos Eduardo Ferreira Pinto told O Estado de Minas newspaper.
The Fundao dam breach caused rivers of mud to descend on nearby villages. Bento Rodrigues was totally destroyed. More than 500 people lost their homes there.
Last week, Brazilian environmental agency Ibama fined Samarco 250m reais.
Ibama described the dam burst as “the worst mining accident in Brazil’s history”.