Five people have been killed in an "horrific" mass shooting at the headquarters of a major beer brand in the American state of Wisconsin, perpetrated by a disgruntled ex-employee.

Heavily armed police swarmed the sprawling Molson Coors Brewing Company campus in Milwaukee after the man opened fire at about 2pm local time (9am NZT).

The several hundred employees received panicked text message and email alerts urging them to hide as the attacker – who had been fired from his job earlier that day – shot at people.

"Find a safe place, active shooter on campus," one of the messages read.

The 51-year-old gunman had been dismissed earlier in the day and "stolen the name tag" of a co-worker in order to gain entry to a packaging warehouse.

He then opened fire with a firearm equipped with a silencer, around the time that a shift change of brewing workers was taking place.

Milwaukee Police chief Alfonso Morales said there were five victims, in what is one of the worst mass shootings in Wisconsin history. The gunman also died by suicide.

Earlier, the city's Mayor Tom Barrett was visibly shocked as he told reporters that "multiple" people were killed in the attack.

"It is a horrible, horrible day for the employees here," Barrett said in a press conference. "It's a very rough day for anyone who is close to this situation.

"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."

Within moments of the first frantic calls to authorities, there was a heavy police presence in the area, along with fire trucks and ambulances.

The Federal Bureau of Investigation was also on the scene and live video showed armour-clad officers with assault weapons.

Despite the presence, it took officers more than two hours to sweep the complex and surrounding streets to ensure there were no other active threats.

The Molson Coors headquarters is a mammoth facility including a mix of corporate offices and brewing facilities. At least 600 people work there.

One worker who was arriving to start her shift as a machine operator told The Washington Post that she received a text message about the shooting.

"I'm feeling horrible," she said from the scene. "These are people we've worked with for years. They're like family. We spend more time with them than we do at home."

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 an active shooter and she was locked in a room with a number of co-workers, the newspaper reported.

A school on the same street was placed in lockdown and students hid in classrooms with the doors barricaded.

One parent, Natasha Williams, told CBS Chicago that she was among many trying to collect their kids from the school when the incident unfolded.

"They told me to go to the coat closet to hide, but then, when it was a code yellow, they told me to go in the auditorium, and wait for our buses and our mums," her son Mario told the outlet.

US President Donald Trump spoke about the incident during a press conference at The White House, extending his condolences to the victims and their families.

"Earlier today, a wicked murderer opened fire at a Molson Coors brewing company plant, taking the lives of (several) people," Mr Trump said.

"Our hearts break for them and their loved ones."

Reaction to the tragedy was swift, with former Congresswoman Gabrielle Giffords – who survived being shot in the head in 2011 - saying she was "absolutely heartbroken".

"And I'm fed up this keeps happening when we know there are laws that would save lives," Giffords said in a tweet.

"The House passed a universal background checks bill one year ago tomorrow. It has bipartisan support. Yet Trump and (Senate Majority Leader Mitch) McConnell won't even try to pass it.

"Every day they fail to act is a day we fail to protect our schools, workplaces, and communities."

"Miller Valley" 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. A massive red Miller sign towers over the complex and is a well-known symbol in Milwaukee, where beer and brewing are intertwined in the city's history.

The facility is also home to corporate customer service, finance, human resources and engineering. Tours take people to underground caves where beer was once stored, a saloon with intricate woodwork, a stein hall with stained-glass windows, a champagne room meeting hall with leaded-glass windows, and an outdoor beer garden that can hold 300 people.

Before Wednesday's shooting, there had been three mass killings nationwide in 2020, with 12 total victims. All have been shootings. In 2019, there were 44 mass killings, with 224 total victims.

- AP and wires