Negotiations over whether Uber must stop its newly launched self-driving car service in San Francisco have concluded without a clear resolution, according to California transportation regulators.

The state has threatened legal action if the ride-hailing company continues to pick up passengers in a handful of cars without having gone through a permitting process. Uber says the cars are exempt from the permit requirement because they have a backup driver behind the wheel who must monitor the car's performance.

State officials had a "positive conversation" Thursday with Uber about "how the company plans to comply with state regulations for self-driving vehicles," Melissa Figueroa, a spokeswoman for California's State Transportation Agency, told The Associated Press. Talks will resume Friday morning, she said.

Uber did not immediately reply to requests for comment.

Meanwhile, the cars could be seen traveling San Francisco's streets Thursday evening.

RED LIGHT WARNING

The two sides met privately in Sacramento the same day that dash cam video posted online showed a self-driving Uber run a red light on Wednesday, the same day the company launched the pilot program with several Volvo SUVs.

On Thursday, Uber said in a written statement that the driver was suspended and attributed the infraction in front of the San Francisco Museum of Modern Art to "human error." That was an apparent reference to the company's policy that employees behind the wheel of the cars must constantly monitor them and be prepared to take over if the technology stops working, or is about to do something dangerous or illegal.

Far from playing defense, Uber offered the driver's failure as evidence of the need to continue pushing ahead a technology that proponents say will one day drive far more safely than humans.

"This is why we believe so much in making the roads safer by building self-driving Ubers," the company said in a written statement, which said the red light-running car was not one of those in its pilot program and was not carrying passengers.

Getting a permit is not a complicated or lengthy process, and regulators would likely approve Uber's application, as they have permits for 20 other companies.

Instead, Uber has insisted it will not apply out of principle, saying its cars do not meet the state's legal definition of an "autonomous vehicle" and therefore do not need a permit.

Though the cars are tricked out with sensors so they can steer, accelerate and brake, and even decide to change lanes, Uber says they are not nearly good enough to drive without human monitoring. And, according to Uber's reading of state law, that means they are not, legally speaking, "autonomous vehicles" that need special state permission.

PUSHING THE LEGAL LIMIT

Pushing legal boundaries is a proud tradition at Uber. During its meteoric rise into a multibillion dollar company, Uber has argued with authorities in California and around the world about issues including driver criminal background checks and whether those drivers should be treated as contractors ineligible for employee benefits.

Both the California Department of Motor Vehicles and its parent transportation agency insist Uber is wrong — and hours after the self-driving service's launch the state sent a letter saying the service was illegal because it lacked the permits.

"If Uber does not confirm immediately that it will stop its launch and seek a testing permit, DMV will initiate legal action," DMV Chief Counsel Brian Soublet wrote the company. He referenced the possibility of taking Uber to court.

San Francisco Mayor Ed Lee joined the chorus of officials denouncing the move, calling it unlawful and ill-advised in a statement. Lee said he was worried about the safety of the city's cyclists and pedestrians, especially with the experiment launching in a week when streets are slick with rain.

Meanwhile, the company is sending another message to California: Other places want us if you don't.

In a blog post Wednesday, the leader of its self-driving efforts, Anthony Levandowski, warned that "complex rules and requirements could have the unintended consequence of slowing innovation" and named several places outside California he characterized as being "pro technology."