PALO ALTO, U.S. -- Toyota Motor on Tuesday announced it will invest an additional $500 million in Uber Technologies, in a move aimed at expanding its partnership with the American ride-hailing company beyond vehicle leasing to the development of self-driving cars.

In a pilot project, a new self-driving car is to be deployed on the Uber ride-sharing network in 2021. The two companies will also consider operating mass-produced autonomous vehicles, as well as the option of bringing in mutually agreed upon third-party operators for the autonomous fleet.

Toyota's Sienna Minivan will serve as the initial platform for realizing what the automaker calls Autono-MaaS, or autonomous mobility as a service. The car will be linked at all times to Toyota's Mobility Services Platform, its core information infrastructure for connected vehicles.

The vehicle will also incorporate both Uber's autonomous driving system and Toyota's automated safety support system. Both systems "will independently monitor the vehicle environment and real-time situation, enhancing overall vehicle safety for both the automated driver and the vehicle," Gill Pratt, CEO of the Toyota Research Institute, said in a statement.

Uber halted its own development efforts in the technology after one of its vehicles killed a pedestrian in the U.S. state of Arizona this March. By strengthening ties with technologically advanced Toyota, Uber is expected to recommit to the field and take on the likes of Google-affiliated Waymo, which aims to commercialize a self-driving mobility service this year.

Toyota announced a ride-hailing partnership with Uber in 2016. The Japanese group also invested an undisclosed sum believed to total tens of millions of dollars. One arrangement they set up involves Toyota leasing vehicles to Uber drivers who then make their lease payments with money earned from rides.

Based on Toyota's $500 million investment, Uber's valuation would come to $72 billion. Toyota's stake is still expected to remain under 1%. The automaker has been developing new services, such as vehicle maintenance, financing and insurance, by partnering with companies outside the auto industry that hold large amounts of traffic data. Toyota announced this June a $1 billion investment in Singaporean ride-hailing company Grab, the industry leader in Southeast Asia.

"This agreement and investment marks an important milestone in our transformation to a mobility company," Shigeki Tomoyama, Toyota's executive vice president, said in the statement.

Uber chief executive Dara Khosrowshahi said, "Our goal is to deploy the world's safest self-driving cars on the Uber network, and this agreement is another significant step towards making that a reality."

Uber has labored to change its famously abrasive culture in the year or so since Khosrowshahi took over as CEO. His predecessor, Travis Kalanick, resigned in disgrace following serious corporate missteps such as mishandling sexual misconduct. Under Khosrowshahi's leadership, the company appears to be making steady progress toward an initial public offering slated for the second half of 2019.

Nikkei staff writer Eri Sugiura in Tokyo contributed to this report.