Four people were killed when a helicopter crashed in Colusa County, Northern California, US on Sunday, according to local authorities.

The Colusa County Sheriff’s Office said the crash in a remote area north of Sacramento happened at around 1.15 p.m. local time (2015GMT) near Highway 45 at Reservation Road, CBS Sacramento broadcaster reported.

The identities of the victims are unknown yet, it said.

"NTSB is investigating the August 1, 2021, crash of a Robinson R66 near Colusa, California," the US National Transportation Safety Board (NTSB) said on Twitter.