Uber Technologies has removed its self-driving cars from San Francisco streets, halting the autonomous program one week after its launch as the company faced a regulatory crackdown.

The California Department of Motor Vehicles (DMV) said on Wednesday it revoked the registration of 16 Uber self-driving cars because they had not been properly permitted.

Uber said it was not obligated to have a permit because its vehicles require continuous monitoring by a person in the car.

San Francisco was supposed to be Uber's second testing ground for its self-driving cars. The company unveiled its self-driving cars in September in Pittsburgh.

California defines autonomous vehicles as having the capability to drive "without the active physical control or monitoring of a natural person."

Uber has argued that the law does not apply to its cars, which cannot stay in autonomous mode continuously. A driver and an engineer are in the front seats to take over frequently in sticky traffic situations such as construction zones or pedestrian crossings.

Another 20 companies exploring self-driving cars, including Alphabet's Google, Tesla Motors and Ford Motor Co, have obtained California DMV permits for 130 cars.