Apple has unveiled its fastest, thinnest iPhone yet, packing a much larger screen and 4G capability on the fifth version of the device that has helped it become the world's most valuable corporation.

Marketing chief Phil Schiller introduced the phone to a packed auditorium at the Yerba Buena Center in San Francisco on Wednesday.

The new handset is 18 per cent thinner and 20 per cent lighter than the iPhone 4S but runs twice as fast and sports a 4-inch "retina" display and the ability to surf a high-speed 4G LTE wireless network.

It will go on sale Friday from $199, with analysts forecasting sales of ten to 12 million in September alone.