Apple has unveiled a mini version of its iPad in a move which sees the company trying to muscle in on the market for cheaper tablets, currently dominated by Amazon and Google.

The new model has a 7.9 inch screen which is about two-thirds of the size of the original iPad.

It weighs 0.31 kilograms and is as thin as a pencil.

Apple's marketing chief, Phil Schiller, said: "It's not just a shrunken down iPad, it's an entirely new design."

The iPad mini's screen has a resolution of 1024 x 768 pixels, the same as an iPad 2 and around a quarter of the resolution of the third-generation iPad.

Prices in the US start at $329.

The most surprising announcement at Tuesday's keynote speech in California was of an upgrade to the full-size iPad, just six months after launching a new model.

The new fourth-generation iPad will have a better camera, work on more "LTE" wireless networks and contain a processor which is double the speed of those seen in third-generation models.

Both the iPad mini and fourth-generation iPad will use Apple's new Lightning connector, which was first introduced on the iPhone 5 last month.