Apple's iPhone-compatible smart watch enables wearers to answer calls, check emails and access apps without taking their phones out of their pockets. It also tracks fitness, plays music, offers customisable watch faces and comes in different colours and styles.

"Within consumer technology, smart watches are the biggest new category since tablets," said Daniel Matte, US-based analyst for research firm Canalys. Canalys has forecast 20 million Apple Watch shipments for 2015.

"Vendors other than Apple are improving their designs to be more fashionable and of higher quality, but the Apple Watch still has a significant design advantage," said Matte.

A host of smart watches are on the market from major players including Samsung, Sony, LG and Motorola, with Taiwan's Asus launching its new ZenWatch 2 Android Wear watch at Computex.