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Uber has struck a deal with the US National Aeronautical and Space Administration (Nasa) as part of its plans to roll out flying cars in cities around the world.

It would work with the space agency to develop software for managing flying taxi routes, Uber's chief product officer Jeff Holden announced at the Web Summit in Lisbon.

Nasa is yet to comment on the deal.

Experts questioned whether the plan could ever get off the ground.

Uber has been involved with regulatory tussles around the globe over its app-based taxi service, and is hoping to avoid similar rows over its air plans.

"We are very much embracing the regulatory bodies and starting very early in discussions about this and getting everyone aligned with the vision," said Mr Holden.

Uber wants to see widespread use of electric, on-demand air taxis - part-drone, part fixed-wing aircraft - which could be ordered via a smartphone app in the same way as its road-based taxis.

It is hoping to get its airborne ride-sharing service off the ground by 2020, when it plans tests in Dallas and Dubai.

Previously, Nasa has said that it was working with a variety of companies to develop urban air mobility.

The biggest issue is how potentially hundreds of thousands of aircraft could operate alongside existing air traffic control systems.

Prof David Dunn, from the University of Birmingham, said there were many hurdles to overcome.

"Many firms are looking at this and there is a lot of blue sky thinking going on about how we can access the air in ways we haven't done before," he told the ³ÉÈË¿ìÊÖ.

"Technologically it is feasible but that is not to say that it is feasible in terms of opening up airspace. There are problems about how such transport will be policed, insured and registered."

He compared Uber's plans to air carrier Pan Am, which in the 1960s promised to take paying customers to the Moon within a few years

"Uber is associating its brand with the future. This might be more about marketing than a realistic product," he said.

Uber does not plan to build aircraft, but rather the software that would manage the network. It said it would work with Pipistrel Aircraft and a variety of other companies who would build vertical take-off and landing aircraft.

The company also announced that Los Angeles would be the third city to partner in pilots of the technology.

It said it was working with developers Sandstone Properties in the city to build rooftop landing pads on skyscrapers.