Apple has unveiled the third edition of its smartwatch, which features built-in cellular service so it can be used completely independently from the iPhone.

Apple Watch Series 3 was debuted by Jeff Williams, Apple's chief operating officer, during today's keynote event in California.

Its major new feature is the inclusion of built-in cellular service, so users can make and receive calls wherever they are without relying on the iPhone, but using the same mobile phone number.

"Now you have the freedom to go anywhere with your Apple Watch," said Williams. "You can go for a run with just your watch. You can leave your phone if you go to the beach. You can receive a call with just your watch."

The watch will also allow users to stream any Apple Music song from their wrist. The Find My Friends feature will automatically switch to the watch when moved away from the iPhone, and the Siri voice-command tool will speak from the watch, rather than simply displaying text and images.

"To enable these features on the watch, we've packed Series 3 with the most advanced features ever in a watch," Williams said.

The team has packed a dual-core processor, a custom W2 chip for Bluetooth and wifi, and a barometric accelerometer for measuring altitude into the watch.

However, the case for Series 3 is same size as the previous Series 2 design, while its back is just 0.25 millimetres deeper.

To save space, the display acts as the antenna, while an electronic SIM card that is "a fraction of the size" of a nano SIM is integrated inside.

The watch will come with wide selection of cases and bands, including a new design called "sport loop". It will still offer up to 18 hours of battery life.

The launch prices for Apple Watch Series 3 are $399 with cellular, and $329 without, available initially in nine countries on selected carriers. Orders will be taken from 15 September 2017, and shipped starting 22 September 2017.

The Series 1 model, first released in 2014, will still be available for $249.

Also during the Apple Event, taking place 12 September 2017, CEO Tim Cook revealed that since the launch of Series 2 in 2016, usage of the smartwatch has increased by 50 per cent. He also unveiled the anticipated iPhone X, which features facial recognition technology.