A gunman killed five people at Molson Coors' brewing campus in the US on Wednesday before turning the gun on himself, police confirmed.

The assailant was identified as a 51-year-old Milwaukee man who worked at the brewery, but police did not offer a motive for the attack nor did they disclose how the attack unfolded.

Mayor Tom Barrett said it was a "horrible day" and urged residents to stay away from the area.

""There were five individuals who went to work today, just like everybody goes to work, and they thought they were going to go to work, finish their day and return to their families," he said.

"They didn't and tragically they never will."

The attack occurred at a sprawling complex that includes a mix of corporate offices and brewing facilities which is widely known as "Miller Valley", a reference to the Miller Brewing Co. that is now part of Molson Coors.

At least 600 people work at the complex.

In an email sent to employees, Molson Coors CEO Gavin Hattersley expressed a "deep sadness" about the attack.

"Unfortunately, I am devastated to share that we lost five other members of our family in this tragic incident," he said.

Police respond to the shooting at the Molson Coors site. Credit: AP

James Boyles told the Milwaukee Journal Sentinel that his wife, Lasonya Ragdales, works at Molson Coors in the claims department.

She was texting from inside the facility and told her husband that there was an active gunman and she was locked in a room with a bunch of co-workers, the Journal Sentinel reported.

Molson Coors features a 160-year-old brewery, with a packaging center that fills thousands of cans and bottles every minute and a distribution center the size of five football fields.

The facility is also home to corporate customer service, finance, human resources and engineering.