A US man with a large home arsenal was charged on Thursday for allegedly threatening to open fire at a Los Angeles hotel, as the United States reels from recent mass shootings.

The suspect, 37-year-old Long Beach Marriott employee Rodolfo Montoya, allegedly told a co-worker that he was angry and planned to open fire at work on anyone who crossed his path.

Police found an assortment of weapons, including an AR-15 rifle, which Montoya was not authorised to have in the state of California. The weapon has been commonly used to commit a number of US gun massacres.

Authorities additionally found other firearms, hundreds of rounds of ammunition and tactical gear.

"Suspect Montoya had clear plans, intent and the means to carry out an act of violence that may have resulted in a mass casualty incident," Long Beach Police Chief Robert Luna told reporters at a news conference after the arrest.

According to an initial investigation, Montoya, who was a cook, was upset over recent workplace activity having to do with human resources.

His colleague took the threat seriously and told a manager who alerted the police.

Montoya was arrested on Tuesday at his home.

Luna told the hotel manager that he "saved many lives, not only of your employees but any customers that may have been at the Marriott when this guy decided to show up".

The charges come after two mass shootings in the US earlier this month - one in Dayton, Ohio and the other in El Paso, Texas - left a total of 31 people dead.

Following those shootings, President Donald Trump has said he supports expanding background checks but has come out against an assault weapons ban, a measure that is also opposed by the powerful gun lobby, the National Rifle Association.