Is Apple really building an electric car? Obviously, says Elon Musk, founder and CEO of electric automaker Tesla.

"It's pretty hard to hide anything if you hire over 1,000 engineers to do it," Musk told BBC News in an interview posted online Monday.

When Musk was asked if he thought Apple was serious, he responded, "Yeah, I do. It's an open secret."

'Owning a car that is not self-driving in the long term will be like owning a horse.' - Elon Musk, Tesla

He added it's hard to build an electric car, but he encourages more companies to do it, and thinks firms like Apple will make "a compelling electric car."

Reports about Apple's car, allegedly code-named Project Titan, have been circulating for some time, but they got a boost last February when electric-car battery maker A123 Systems sued Apple last February for poaching its top engineers. At that time, more than 60 former Tesla employees had already been hired by Apple.

Reuters reported in August that Apple had not only hired senior engineers with expertise in automated driving technology from Tesla, but also Volkswagen, microchip maker NVidia Corp., Carnegie Mellon University and the Swiss Federal Institute of Technology.

Apple Inc.

The Apple-focused technology website MacRumours reported Friday that Apple has registered the domain names, and (Yuya Shino/Reuters)

The Wall Street Journal reported in September the Apple car will ship in 2019, and Apple recently met with California officials about the state's self-driving car regulations.

On Friday, the Apple-focused technology website MacRumours reported Apple has registered the domain names, and

Electric everything, except rockets

Musk, also founder and CEO of rocket and spacecraft-maker SpaceX, told BBC News he thinks all transport except rockets will go fully electric and in the long term, and no one will buy a car unless it can drive itself.

"Owning a car that is not self-driving in the long term will be like owning a horse."

Tesla has recently been releasing software updates that give its cars increasingly sophisticated semi-autonomous driving abilities. The latest, beamed to Tesla cars over the weekend, allows them to park themselves without a driver inside and puts new speed limits on the cars' semi-autonomous Autopilot mode.

While Tesla's current cheapest model sells for a minimum of $100,000 – firmly in the luxury vehicle range – Musk says the company aims to have a more affordable, high-volume car called the Model 3 in production by the end of next year.