Four people, including a child, were shot and killed Wednesday evening at an office building in Southern California, police said.

It is the third such shooting in the United States in weeks, with 18 people killed in two separate gun violence incidents in March.

The shooter, whose motivations are so far unknown, sustained a gunshot wound and was taken to hospital, police lieutenant Jennifer Amat said, adding the suspect was in a critical condition.

She continued that officers were still working to determine whether the wound was self-inflicted or a result of an exchange of fire with police.

Police did not release any more information about the victims, but said that a fifth individual -- a woman -- had been hospitalized and was in a critical condition.

"The situation has been stabilized and there is no threat to the public," the Orange Police Department said in a post to their Facebook page.

The incident began around 5:30 pm local time on the upper floor of a small office building in the city of Orange.

The block houses businesses including a counseling service, an insurance company, a financial consulting firm and a phone repair shop, local media reported.

Police and the suspect exchanged fire, according to the Los Angeles Times.

One man, who asked not to be identified speaking to local TV channel KTLA5, described the situation as "terrifying".

Cody Lev told local media The Orange County Register that they heard three loud pops, followed by silence and then sirens.

"I can tell you that we haven't had an incident like this in the city of Orange since 1997," Amat said, referring to a mass shooting event that saw four people shot dead.

She added: "It's just such a tragedy for the victims, their families, our community and our police department."

California governor Gavin Newsom posted on Twitter: "Horrifying and heartbreaking. Our hearts are with the families impacted by this terrible tragedy tonight."

"I'm deeply saddened by reports of a mass shooting in Orange County, and I'm continuing to keep victims and their loved ones in my thoughts as we continue to learn more," US Representative Katie Porter from California also tweeted.

It comes after two other high-profile mass shootings earlier this month, which set off a renewed debate about gun control measures in the United States.

On March 22, 10 people were killed in a shooting at a grocery store in Boulder, Colorado, less than a week after a man shot and killed eight people, including six women of Asian descent, at spas in Atlanta, Georgia.