The break caused a sea of muddy sludge that buried the dam's cafeteria where hundreds of workers were eating lunch.
Rescue teams used earth-moving machinery at the site near the town of Brumadinho, in Minas Gerais state.
Governor Romeu Zema said there was little chance of finding people alive. Nine people have been confirmed dead.
It is not clear what caused the collapse of the dam, owned by Brazil's largest mining company, Vale.
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