The top US auto safety regulator on Tuesday said it had opened an investigation into Google-owned Waymo after reports of 22 incidents with its self-driving technology.

The National Highway Traffic Safety Administration's (NHTSA) Office of Defects Investigation said the problems took place in vehicles equipped with Waymo's fifth generation automated driving system.

The preliminary probe into the robotaxi company will cover the firm's 444 vehicles, according to documents posted on the agency's website.

Waymo vehicles are available to customers in San Francisco and Phoenix, with Los Angeles and Austin expected to come online next.

The NHTSA said reports included "collisions with stationary and semi-stationary objects such as gates and chains, collisions with parked vehicles, and instances in which the cars appeared to disobey traffic safety control devices."

The investigation will evaluate the technology's ability to avoid collisions with objects and vehicles, the office said.

The NHTSA on Monday opened a similar inquest into Amazon-owned Zoox after receiving two reports that the company's autonomously-operated Toyota Highlanders were rear-ended after they pulled on the brakes too quickly.

Zoox has been testing its robotaxis in Las Vegas, Nevada, and Foster City, California.

Earlier this year, Zoox expanded its deployment to include faster speeds, nighttime driving and driving in light rain conditions.

The NHTSA has also launched probes into Ford and Tesla as it investigates the wider deployment of autonomous technology.

GM subsidiary Cruise in April said it plans to get its self-driving cars back on the road after suspending robotaxi service late last year due to safety concerns.