Three people have been killed after two firefighting helicopters collided in Southern California while fighting a blaze in Riverside County, emergency officials said.

One of the helicopters landed safely following the incident on Sunday.

Cal Fire Southern Region chief David Fulcher said during a news conference: “Unfortunately, the second helicopter crashed and tragically all three members perished which included one Cal Fire Division chief, one Cal Fire captain and one contract client pilot.”

The victims have not yet been identified.

Cal Fire and Riverside County Fire Department resources were dispatched to a reported structure fire near the intersection of Broadway Street and South Ronda Avenue in the community of Cabazon, Mr Fulcher said.

Shortly after the arrival of the first engine company, the fire was reported to have extended into the vegetation and a full wildland fire dispatch was initiated, which included fixed-wing and rotary-wing aircraft.

While battling the blaze, the two helicopters collided.

The crash caused an additional four-acre (1.6-hectare) fire, which was later extinguished.

“Although this was a tragic event, we are also thankful today that it wasn’t worse,” Mr Fulcher said.

“The individuals in the first helicopter were able to able to land safely without incident and no-one else was hurt.”

The helicopter pilot was performing work under contract with the California department of forestry and fire protection, fire captain and spokesman Richard Cordova said on Sunday evening.

The crash is being investigated by the US National Transportation Safety Board.