Authorities have ordered thousands to evacuate from a dam at risk of giving way in south-eastern Brazil, just days after a nearby dam collapsed.
At least 37 have been killed and more than 250 others are feared dead in Friday’s disaster at an iron ore mine near the town of Brumadinho.
Rescue operations have been suspended after officials warned of dangerously high water levels at the second dam.
Both dams at the mining complex belong to Vale, Brazil’s largest mining firm.
Warning sirens rang out at 5:30 local time (7:30 GMT) over Brumadinho, which is home to some 39,000 people.
All the town’s bridges have been ordered to close, while residents rush to higher ground.
“With bags on their backs, everything they could grab, children, old people climbing up, it was total despair,” one man told the BBC.
The rupture of the dam on Friday caused a sea of muddy sludge to bury the site’s cafeteria, where workers were eating lunch, and at least one bus that carried employees, before engulfing buildings, vehicles and roads.
On Saturday, emergency services used helicopters and earth-moving machinery in the search for survivors.
Rescue efforts were suspended on Sunday over fears of the second dam bursting, according to reports.
Some 366 people have been rescued, Minas Gerais state fire department says. Of those, 23 have been taken to hospital.
Vale said it could not reach 252 of its employees and contractors. Residents and guests at an inn were also reported missing.
Eight of the victims have been identified.
Search efforts have been hampered by the difficult access to the area, the BBC’s Julia Carneiro in Brumadinho reports.
Even retrieving the bodies will be complicated because they are thought to be under a thick layer of mud, our correspondent adds.
A bus that was carrying workers was found but it was unclear how many people were inside. Three people who were stranded in a pick-up truck were rescued alive.
President Jair Bolsonaro, who flew over the disaster area in a helicopter on Saturday, tweeted that it was hard not to get “emotional” after seeing the scale of the devastation.
He said he had accepted an offer by Israel to send search equipment that could find people buried in the mud.
State prosecutors announced on Sunday they had frozen a total of 11bn reais ($2.9bn; £2.2bn) of Vale’s assets. A judge had earlier frozen 6bn reais from its accounts to help fund recovery works and handle damages.
Relatives of the missing gathered at a warehouse set up by Vale criticised the lack of information.
“I want to know the truth. There’s a disregard for everyone here,” 43-year-old Sirlene do Carmo Januário told the BBC – her missing 23-year-old son, Rangel, worked at a company’s building that was hit by the sludge.
Earlier, state Governor Romeu Zema said the chances of finding more survivors were slim. “We’re likely to just be recovering bodies.”
The collapse comes just over three years since a dam burst in Mariana, also in Minas Gerais, killing 19 people, in what was considered Brazil’s worst environmental disaster.