A dam collapse at an iron-ore mine in southeast Brazil has left at least seven people dead and some 150 others missing, officials say.The break caused a sea of muddy sludge to spread across rural areas of Brumadinho, in Minas Gerais state, burying buildings and vehicles on Friday, the BBC reported.It is not clear what caused the dam, owned by Brazil's largest mining company, Vale, to break.Many of those missing were workers who were having lunch in the dam's cafeteria that was buried by the sludge.The dam near the Feijao iron ore mine burst its barrier at around lunchtime, flooding another dam down below.Leaked tailings from the mine spread into the nearby community of Vila Forteco, close to Brumadinho. Residents in low-lying areas have been evacuated. Emergency teams rescued scores of trapped people by helicopter," Brumadinho Mayor Avimar de Melo said, adding, "We do not have more details because everything is happening very fast.""Our main worry now is to quickly find out where the missing people are," he told television channel GloboNews.Some 100 rescuers are searching for the missing with 100 more expected to join them on Saturday, the BBC said.On Twitter, President Jair Bolsonaro called it a "serious tragedy" and said he would visit the region later on Saturday. The ministers for the Environment, Mines and Energy, and Regional development have already set off to the scene.Built in 1976, the dam was one of several in the area and it was used to hold residue from the mine. With a capacity of 12 million cubic metre the site has been lying inactive for three years, according to Vale. It is not yet known how much waste was released.The accident comes just over three years since a dam burst in another town in Minas Gerais, killing 19 people.