Police Chief Alfonso Morales speaks to the media following a shooting at the Molson Coors Brewing Co campus on Wednesday in Milwaukee, Wisconsin, the US. Six people, including the gunman, were reportedly killed. Photo: AFP

A gunman killed five employees of 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.

News reports said the gunman was an employee. 

Mayor Tom Barrett said five other people, all workers at the facility, were killed.

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.

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.

Molson Coors, a Canadian-US company, said it was working with the police department, adding: "Our top priority is our employees." 

The Milwaukee Journal Sentinel quoted the husband of a brewery employee, Lasonya Ragdales, as saying she was informed by the company of an active shooter on the premises, and was texting him while locked in a room with co-workers.

Nearby schools were also placed on lockdown.

Known until last year as MillerCoors, the brewer announced in October that it would undergo a large restructuring and officially changed its name to Molson Coors Brewing Co.

It also moved its North American headquarters office from Denver to Chicago and shifted hundreds of corporate office jobs to Milwaukee.

The company was expected to cut 400 to 500 jobs throughout the organization during the restructuring.

The scene of the shooting is known locally as the "old Miller" brewing company, Morales said.

It was the latest in a long list of gun-related violence in the US, which saw a record 417 mass shootings in 2019, according to the research group Gun Violence Archive.

Democratic presidential hopefuls weighed in on the latest case.

"May we find the strength and will to act against gun violence, and never accept the unacceptable," said Pete Buttigieg.