Tesla CEO Elon Musk has admitted that a "massive effort" is required for Tesla driverless cars to get to "99.9999% safety."

In a tweet, Musk said intersections with lots of traffic lights and shopping mall parking lots are among the technology's biggest challenges.

His comments come as at least four people have been killed while using Tesla Autopilot. Tesla maintains that vehicles that have Autopilot engaged have lower accident rates.

Visit Business Insider's homepage for more stories.

Tesla CEO Elon Musk has admitted that the company has got a lot of hard work to get through if it's going to launch a robot taxi network in 2020.

In a tweet on Sunday, Musk said "massive effort" is required to get the technology to "99.9999% safety." He also highlighted two areas where the software needs to improve.

Read more: Tesla is taking direct aim at Uber and Lyft with claims it plans to roll out 1 million robo-taxis by next year — but the plan was lacking details

"Intersections with complex traffic lights & shopping mall parking lots are the two biggest software challenges," Musk explained.

The tech billionaire set out plans in April to "have over 1 million robo-taxis on the road" by 2020. In his tweets on Sunday, he was more specific, saying it would be "by end of" next year.

Tesla Model 3. Matthew DeBord/BI

Tesla will want to finesse the technology after its Autopilot feature has been controversial for the company. Over the past few years, at least four people have been killed while using this system, raising questions over whether the technology is safe.

Despite this, Tesla maintains that vehicles with Autopilot engaged have lower accident rates than those that do not. "Our data shows that, when used properly by an attentive driver who is prepared to take control at all times, drivers supported by Autopilot are safer than those operating without assistance," a Tesla representative said in a recent statement to the press.