SAN FRANCISCO: Google executive David Drummond left the Uber board of directors Monday as competition revs up between the companies over self-driving cars.

"I recently stepped down from Uber's board given the overlap between the two companies," Google chief legal officer Drummond said in a statement.

Drummond noted that Google remains an "enthusiastic investor" in the on-demand ride-sharing service.

Uber co-founder and chief executive Travis Kalanick said Drummond had been a "sage advisor and great personal friend,”" and adding that he was looking forward to continued cooperation between the companies.

Google parent Alphabet and San Francisco-based Uber have both been working on getting self-driving cars on roads, with their relationship evidently getting bumpier as efforts gain traction.

Uber recently announced plans to deploy driverless cars for its ride-sharing services in Pittsburgh this month, pushing the envelope for the use of self-driving technology.

Uber said the program would begin with the cars carrying company "co-pilots," engineers and safety personnel.

At the same time, Uber also announced two other moves to further solidify itself as a trailblazer in driverless cars.

It established a $300 million venture with Chinese-owned, Sweden-based Volvo Cars to develop self-driving cars for sale by 2021.

And Uber is buying Otto, a San Francisco startup developing self-driving commercial trucks.

For the past two years, Uber has made a strong push into developing self-driving car technology.

Uber and Volvo were two of the founding members of a coalition unveiled in April to push for a unified US legal code on self-driving cars ─ a group that also includes Google, carmaker Ford and Uber rival Lyft.

Autonomous cars are among big-vision ideas pursued by Google's X Lab.

A car industry executive was hired last year to turn it into its own company at Google parent Alphabet. Google has driven its autonomous cars some 1.5 million miles (2.4 million kilometres) with only some minor dust-ups.

In May, the company announced plans for its self-driving car program to put down roots in the Detroit area with a technology centre.

The facility will house engineers and others testing vehicles provided by Fiat Chrysler Automobiles, Alphabet said at the time.

The collaboration with Fiat Chrysler marked the first time the internet giant has worked directly with an automaker to build self-driving vehicles.

The tech giant began testing its autonomous driving technology in 2009 using a Toyota Prius equipped with Google equipment.

It now has some 70 vehicles, including Lexus cars, adapted by Google in addition to its in-house designed cars unveiled in 2014.