A mid-air collision took place between two small aircraft in the US state of California on Thursday which resulted in multiple casualties, officials said.

The incident took place in the city of Watsonville after two planes attempted to land at the local airport.

"Multiple agencies responded to Watsonville Municipal Airport after 2 planes attempting to land collided. We have reports of multiple fatalities," the city officials said in a statement posted via Twitter.

An investigation into the incident is currently underway.

More information will become available throughout the day, the statement said.