Apple is working on the "autonomous systems" behind driverless cars, Chief Executive Tim Cook said in an interview published by Bloomberg on Tuesday, in his most detailed comments about the company's secretive plans.

The technology giant has been tight-lipped on any of its autonomous vehicle plans, despite numerous reports that it has been researching them under codename "Project Titan".

Cook's comments illustrate that Apple is working on the software behind the vehicle.

"We're focusing on autonomous systems. And clearly, one purpose of autonomous systems are self-driving cars. There are others," Cook said in an interview with Bloomberg on June 5, that appears to have been published Tuesday.

"And we sort of see it as the mother of all AI projects. It's probably one of the most difficult AI projects actually to work on and so autonomy is something that's incredibly exciting for us, but we'll see where it takes us."

Cook made clear that the company is not ready to say what exactly they will do from a "product point of view".