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.