Amazon upped the ante in the tablet wars Wednesday, unveiling upgraded Kindles with a boost in power while adding a "Mayday" button for free, live technical support.

The US online retail giant said the Kindle Fire HDX tablets would include an improved display that goes "beyond HD with exceptional pixel density" and a new processor that provides "three times the processing power" of the current version of Kindles.

The new tablet is offered in a seven-inch (17.8-centimeter) version that competes with smaller devices like Apple's iPad mini, and an 8.9 inch (22.6 centimeter) model closer to the larger iPad and similar units.

"It's been just two years since we introduced the first Kindle Fire, and the team is innovating at an unbelievable speed," said Jeff Bezos, Amazon's founder and chief executive.

"We've worked hard to pack this much hardware, innovation and customer obsession into these prices."

The smaller tablet will start at $229, with pre-orders starting Wednesday for shipping October 18.

Amazon cut the price of the current generation of Kindles to as low as $139, from the launch price of $199 last year.

The larger Kindle Fire HDX starts at $379, and will begin shipping on November 7.

Amazon also introduced what it called a "revolutionary" tech support system to help people learn to use the Kindle.

"Simply tap the Mayday button in Quick Settings, and an Amazon expert will appear on your Fire HDX and can co-pilot you through any feature by drawing on your screen, walking you through how to do something yourself or doing it for you -- whatever works best," an Amazon statement said.

It aims to provide a response within "15 seconds or less" and free tech support will be available 24 hours a day, 365 days a year.

Amazon has not released sales figures for the Kindle tablets, but analysts say it is among the biggest rivals to the market-leading iPads, with a US market share that may have been as high as 22 percent last year.

Earlier this year, Kindle sales were expanded to 170 countries from just a handful.

The Kindles use what Amazon calls a Fire OS operating system, which builds on Google's Android system with added cloud services, a content-first user interface, built-in media libraries, productivity apps and low-level platform enhancements to "integrate Amazon's digital content and improve performance."

The new devices will have the Fire OS 3.0 "Mojito" system.

Bezos, speaking on CNN, acknowledged that the company does not make a profit from Kindle sales.

"Our approach is premium products at non-premium prices. So, we sell the hardware at breakeven," he told CNN's "New Day."

"So, we don't try to make any money when we sell this hardware and we hope to make money when people use the devices not when they buy the devices.

"And so, that's a very different approach from most companies. Most companies are building quite a bit of profit into the sale of these devices."

The news came just two days after Microsoft announced upgrades to its Surface tablets, which have failed to gain traction as a device marketed as a substitute for both a tablet and a laptop computer.

Yankee Group analyst Carl Howe said the tech support feature could be a selling point.

"Kudos to Amazon for creating this unique support service for its Kindle Fire tablets," Howe wrote in a blog post.

"Tablets are now mature enough that they are reaching consumers who aren't technology-savvy early adopters, and those consumers need more help than previous Kindle buyers. Amazon's new Mayday service will encourage those consumers to love their new Kindle Fires and buy more Amazon content."

Forrester Research analysts Sarah Rotman Epps said Amazon "is delivering a lot of value for the price."

"Tablets are a maturing category, but Amazon has thoughtfully created reasons for more consumers to buy and stick with the Kindle Fire," she said.

"The 'Mayday' customer service feature is unique to Amazon, and will be a huge help to mainstream users of the device. Your mom won't have to call you for tech support anymore."