Six people have been killed – including a gunman – in a shooting at the Molson Coors Brewing headquarters in the US state of Wisconsin.

The gunman has been identified as a 51-year-old Milwaukee man, who also worked at the company.

Milwaukee Mayor Tom Barrett said it was an “unspeakable tragedy” for the city.

“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. They didn’t – and tragically they never will,” he said.

“It is a horrible, horrible day for the employees here. It’s a very rough day for anyone who is close to this situation.”

In addition to the six dead, other workers were injured.

Wisconsin Lieutenant Governor Mandela Barnes said it was the scene of “another American tragedy”.

“A senseless American tragedy, and one that shouldn’t have had to happen,” Mr Barnes said.

Tweet from @MilwaukeePolice

Earlier, the local newspaper, the Journal Sentinel, reported that brewery employees had been notified by email that an active shooter was in or near a second-floor stairwell in a building at the sprawling complex on Wednesday afternoon (local time).

At least 600 people work at the complex, which is widely known in the Milwaukee area as “Miller Valley,” a reference to the Miller Brewing Co. that is now part of Molson Coors.

Molson Coors spokesman Martin Maloney would say only that there was an “active situation”.

There was a heavy police presence in the area, along with fire trucks and ambulances. Live video showed an officer in body armour getting an assault rifle out of car.

Emergency crews continued to arrive at the scene nearly two hours after the initial call to police.

The FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were among the agencies responding.

Milwaukee police asked that people stay clear of the area and that no helicopters or drones be flown over the site for “officer safety purposes”.

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 building and told her husband that there was active shooter and she was locked in a room with co-workers, the Journal Sentinel reported.

From the White House, US President Donald Trump sent his condolences to those involved in the deadly attack.

“Our hearts break for them and their loved ones,” he said.

“We’ll be with them, and it’s a terrible thing, a terrible thing.”

-with AAP