A gunman killed five co-workers at one of America's best-known breweries on Wednesday before turning the weapon on himself in the latest burst of mass gun violence in the US.

More than 1,000 employees were at the Molson Coors brewing complex in Milwaukee, Wisconsin when the early-afternoon tragedy occurred, the city's police chief Alfonso Morales told reporters.

He said officers found the suspect, a 51-year-old local man, dead from an apparently self-inflicted gunshot wound.

Morales later said the suspect was a Molson Coors employee.

Mayor Tom Barrett said five other people, all workers at the facility in the northern US state's biggest city, were killed.

"They thought they were gonna go to work, finish their day, and return to their families," Barrett said at the press conference.

President Donald Trump earlier gave the first official word of the toll.

"A wicked murderer opened fire at a Molson Coors brewing company plant, taking the lives of five people, a number of people wounded, some badly wounded," Trump said at a press conference about the new coronavirus.

US media including ABC News and the local Fox affiliate reported the shooter had been fired earlier in the day from the beer giant, which owns the Coors and Miller brands.

The local CBS affiliate said the shooter appeared to have stolen the nametag of another employee, then returned to the office complex with a gun. But The New York Times quoted Representative Gwen Moore, a Democrat whose district includes Milwaukee, as saying the gunman was an employee who was in uniform.