Seattle - Amazon.com Inc unveiled two new high-definition tablets on Tuesday with a unique on-screen helpdesk feature it hopes will give it an advantage over devices from rivals Apple Inc and Google Inc.

The new Kindle Fire HDX tablets feature what Amazon has called the 'Mayday Button', which instantly brings up a video chat with a tech-support representative who can tell a user how to operate the device or even do it for them remotely.

Amazon said the service is free for HDX customers, is available at all times, and is aiming for a 15-second maximum response time. Users can move the video box around the screen as it suits them and they cannot be seen by the Amazon representative.

The world's largest Internet retailer takes a different approach to the tablet market than Apple, selling its Kindle devices at close to cost then profiting off the sale of digital content such as video and music, or physical goods like books from its website. Since Amazon took the plunge into the then-nascent tablet market with the Kindle Fire in 2011, the devices have proven to be effective vending machines for purchases.

It is unclear how the up-close-and-personal support feature will affect the underlying cost of supporting the Kindle.

"This is the kind of feature that we are well-suited to do," said Amazon Chief Executive Jeff Bezos, showing off the new tablets to reporters at Amazon's Seattle headquarters on Tuesday. "Many of the things we've done marry together high-tech with heavy lifting. This is one of those things."

Amazon would not say how many support staff were behind the Mayday service, but Bezos said the company was training "thousands" and was prepared to hire more people if needed.

"We'll be ready for Christmas morning, which is always a very big tech support day for us," said Bezos.

The new tablets, one with a 7-inch (18cm) screen and one with an 8.9-inch screen, are lighter and more powerful than the last Kindle HD line and appear to be aimed at Apple's market-leading iPad. Amazon's new HDX tablets come in 16GB, 32GB and 64GB storage sizes.

The smaller Kindle Fire HDX tablet starts at $229 and the bigger tablet starts at $379, both for 16GB wifi-only models. By comparison, Apple's 16GB wifi iPad mini starts at $329, and its 16GB full-sized wifi iPad starts at $499.

Amazon is taking pre-orders immediately for wifi-only models, with shipping scheduled for October for the 7-inch tablet and November for the 8.9-inch tablet. Wireless 4G versions of both will also be available, for $100 extra, later this year.

In addition, Amazon is selling an updated version of its Kindle Fire HD for $139, down from $199 for the last generation.

Together, Amazon is hoping the new offerings will give it a bigger share of the exploding tablet market, currently led by Apple's iPad, followed by Samsung's Galaxy range of tablets running Google's Android system.

To promote profitable cross-selling, Amazon has extended its popular 'X-Ray' feature, which now allows users to buy music featured in TV shows and films at the touch of a button.

The new tablets run on the latest version of Amazon's Fire OS, codenamed 'Mojito', which is its own operating system based on Google's Android.