Uber said it is suspending self-driving car tests in all North American cities after a fatal accident.
A 49-year-old woman was hit by a car and killed as she crossed the street in Tempe, Arizona.
While self-driving cars have been involved in multiple accidents, it is thought to be the first time a self-driving car has been involved in a fatal pedestrian collision.
Uber chief Dara Khosrowshahi said the death was “incredibly sad news”.
“We’re thinking of the victim’s family as we work with local law enforcement to understand what happened,” he said in a tweet.
Police said the accident happened Sunday night while the car was in the autonomous mode. A human monitor was also behind the wheel.
Police said the woman, Elaine Herzberg, had not been using a pedestrian crossing. She was taken to a local hospital, where she died.
The US National Highway Traffic Safety Administration and the National Transportation Safety Board said they were sending teams to Tempe.
Anthony Foxx, who served as US Secretary of Transportation under former President Barack Obama, called the accident a “wake up call to the entire AV industry and government to put a high priority on safety.”
Uber started testing driverless cars in Pittsburgh in 2016. The ride-hailing firm has also been testing driverless cars in San Francisco, Pittsburgh, Toronto and the Phoenix area, which includes Tempe.
The death comes a year after Uber took its self-driving cars off the road following an accident that left a Volvo SUV on its side in Arizona. The programme was later reinstated.
Consumer Watchdog, an advocacy group that has called for tougher rules for autonomous cars, called the accident a “tragedy we have been fighting years to prevent”.
“We hope our calls for real regulation of driverless cars will be taken seriously going forward by Silicon Valley and the Trump Administration,” the group wrote on Twitter.