A top US safety watchdog slammed Tesla for not implementing key recommendations to safeguard the automaker's driver assistance programs, in a letter Monday to chief executive Elon Musk.

Jennifer Homendy, chair of the National Transportation Safety Board, said she was "deeply concerned" at Tesla's "inaction" four years after the watchdog agency recommended steps following a crash in Florida that killed the Tesla driver.

In a September 2017 report on the incident a year earlier, the NTSB concluded that Tesla's driver assistance system was prone to being employed on roads for which it was not designed. Tesla's program also failed to detect signs the driver was disengaged.

The agency, which makes recommendations and does not hold regulatory authority over automakers, urged Tesla to incorporate safeguards to limit the system to areas for which it was intended and to alert the driver when he or she became disengaged.

A top auto safety watchdog slammed Tesla over inaction to the agency's safety recommendations
A top auto safety watchdog slammed Tesla over inaction to the agency's safety recommendations AFP / SAUL LOEB

The other five automakers that received the NTSB's recommendations responded and outlined the steps they were taking.

"Tesla is the only manufacturer that did not officially respond to us about the recommendations," Homendy wrote.

Homendy described a second fatal crash in California in 2018 that also took place in a roadway not meant for the driver assistance system and with an operator who was disengaged.

"Our crash investigations involving your company's vehicles have clearly shown the potential for misuse requires a system design change to ensure safety," Homendy said.

"If you are serious about putting safety front and center in Tesla vehicle design, I invite you to complete action on the safety recommendations we issued to you four years ago."