Amazon.com is raising the stakes in the e-book war by adding multimedia capabilities to its downloadlable application for Apple devices.

An update to the Kindle app for Apple's iPhone, iPad and iPod Touch will allow e-book authors and publishers to include video and audio, the online bookseller said Monday.

The company introduced 13 e-books with audio or video to coincide with the launch, and said more are on the way. Rose's Heavenly Cakes, for example, includes video tips on preparing baked goods while Bird Songs has audio of actual bird calls.

"This is just the beginning — we look forward to seeing what authors and publishers create for Kindle customers using the new functionality of the Kindle apps," the company said in a statement.

The introduction of new features for Apple devices follows a price cut on Amazon's own e-reader, the Kindle, last week. Amazon dropped the price of its Kindle to $189 US to match a similar move by U.S. bookseller Barnes & Noble, which cut the price of its Nook e-reader the same day.

Sales expected to skyrocket

Both the Kindle and Nook are e-ink devices, which means they have screens that simulate the look of paper and are better for reading than Apple's gadgets, which use LCD displays. LCD screens use more power and refresh more frequently, so they are harder on the eyes than e-ink displays.

E-ink displays, however, are limited in their multimedia functionality and cannot play video.

Most analysts expect e-reader sales to skyrocket this year, amounting to between six million and 14 million units in the United States alone. Earlier this year, the Yankee Group estimated that more than 19 million e-readers will be sold in the United States by 2013.

The Kindle and Apple's products are also competing with Kobo, the e-book reader from Indigo Books in Canada.

Kobo chief executive Michael Serbinis has predicted that the price of e-book readers will fall to less than $100 by the end of this year.