Brazilian officials on Sunday suspended the search for potential survivors of a dam collapse that has killed at least 40 people amid fears that another nearby dam owned by the same company was also at risk of breaching.

Authorities were evacuating several neighbourhoods in the southeastern city of Brumadinho that were within range of the B6 dam owned by the Brazilian mining company Vale. There was no immediate word on how many people were evacuated.

“Leave here, this is at risk!” police officials told firefighters in a lower-lying area. “Within a little while, more mud will fall.”

The firefighters had been working to extract a cow found alive in the mud, but they pulled back on the order of police, leaving the animal.

While the ground search was stopped, helicopters continued to fly over the area, possibly so they would not be hit if another collapse happened.

Caroline Steifeld, who was evacuated, said she heard warning sirens on Sunday, but no such alert came on Friday, when the first dam collapsed.

“I only heard shouting, people saying to get out. I had to run with my family to get to higher ground, but there was no siren,” she said, adding that a cousin was still unaccounted for.

Even before the latest setbacks, hope that loved ones had survived a tsunami of iron ore mine waste from Friday's dam collapse in the area was turning to anguish and anger over the increasing likelihood that many of the hundreds of people missing had died.

Company employees at the mining complex were eating lunch on Friday afternoon when the first dam gave way. By Saturday night, when authorities called off rescue efforts until daybreak, the dam break toll stood at 40 dead with up to 300 people estimated to be missing.

All day Saturday, helicopters flew low over areas encased by a river of mud and mining waste as firefighters dug frantically to get into buried structures.

“I'm angry. There is no way I can stay calm,” said Sonia Fatima da Silva, as she tried to get information about her son, who had worked at Vale for 20 years. “My hope is that they be honest. I want news, even if it's bad.”

Da Silva said she last spoke to her son before he went to work on Friday, when around midday a dam holding back mine waste collapsed, sending waves of mud for kilometres (miles) and burying much in its path.

She was one of scores of relatives in Brumadinho who desperately awaited word on their loved ones.

Romeu Zema, the governor of Minas Gerais state, said by now most recovery efforts will entail pulling out bodies.

The flow of waste reached the nearby community of Vila Ferteco and an occupied Vale administrative office. It buried buildings to their rooftops and an extensive field of the mud cut off roads.

Some residents barely escaped with their lives.

“I saw all the mud coming down the hill, snapping the trees as it descended. It was a tremendous noise,” said a tearful Simone Pedrosa, from the neighbourhood of Parque Cachoeira, 5 miles (8 kilometres) from where the dam collapsed.

Pedrosa, 45, and her parents dashed to their car and drove to the highest point in the neighbourhood.

“If we had gone down the other direction, we would have died,” Pedrosa said.

“I cannot get that noise out of my head,” she said. “It's a trauma ... I'll never forget.”

In addition to the 40 bodies recovered as of Saturday night, 23 people were hospitalised, according to the Minas Gerais fire department.

There had been some signs of hope earlier on Saturday when authorities found 43 more people alive.

The company said on Saturday that while 100 workers were accounted for, more than 200 workers were still missing.

Fire officials at one point estimated the total number at close to 300.

Vale CEO Fabio Schvartsman said he did not know what caused the collapse.

For many, hope was evaporating.

“I don't think he is alive,” said Joao Bosco, speaking of his cousin, Jorge Luis Ferreira, who worked for Vale.

“Right now, I can only hope for a miracle.”

Vanilza Sueli Oliveira described the wait for news of her nephew as “distressing, maddening.” “Time is passing,” she said. “It's been 24 hours already. ... I just don't want to think that he is under the mud.”

The rivers of mining waste also raised fears of widespread environmental contamination and degradation.

According to Vale's website, the waste, often called tailings, is composed mostly of sand and is non-toxic.

However, a UN report found that the waste from a similar disaster in 2015 “contained high levels of toxic heavy metals.”

Over the weekend, state courts and the justice ministry in the state of Minas Gerais froze about $1.5 billion from Vale assets for state emergency services and told the company to report on how they would help the victims.

Brazil's Attorney General Raquel Dodge promised to investigate the mining dam collapse, saying “someone is definitely at fault.” Dodge noted there are 600 mines in the state of Minas Gerais alone that are classified as being at risk of rupture.

Another dam administered by Vale and Australian mining company BHP Billiton collapsed in 2015 in the city of Mariana in the same state of Minas Gerais, resulting in 19 deaths and forcing hundreds from their homes.

Considered the worst environmental disaster in Brazilian history, it left 250,000 people without drinking water and killed thousands of fish. An estimated 60 million cubic metres of waste flooded nearby rivers and eventually flowed into the Atlantic Ocean.

Sueli de Oliveira Costa, who hadn't heard from her husband since Friday, had harsh words for the mining company.

“Vale destroyed Mariana and now they've destroyed Brumadinho,” she said.

The Folia de S.Paulo newspaper reported on Saturday that the dam's mining complex was issued an expedited license to expand in December due to “decreased risk.” Conservation groups in the area alleged that the approval was unlawful.

On Twitter, new Brazilian President Jair Bolsonaro said his government would do everything it could to “prevent more tragedies” like Mariana and now Brumadinho.

The far-right leader campaigned on promises to jump-start Brazil's economy, in part by deregulating mining and other industries.

Environmental groups and activists said the latest spill underscored the lack of environmental regulation in Brazil, and many promised to fight any further deregulation.

Marina Silva, a former environmental minister and presidential candidate, toured the area on Sunday. She said such tragedies should be deemed “heinous crimes,” and that Congress should bear part of the blame for not toughening regulations and enforcement.

“All the warnings have been given. We are repeating history with this tragedy,” she told The Associated Press. “Brazil can't become a specialist in rescuing victims and consoling widows. Measures need to be taken to avoid prevent this from happening again.”