The US Author's Guild has warned that the new audio read-aloud feature of Amazon's latest Kindle electronic book reader could pose a "significant challenge" to the publishing industry.

"We're studying this matter closely and will report back to you," the guild said in a message to its members on its website, authorsguild.org.

Amazon unveiled the Kindle 2 at a press conference in New York on Monday.

Besides being slimmer and faster than the previous model, the Kindle 2 also has a "text-to-speech" feature that reads a book aloud.

"This presents a significant challenge to the publishing industry," the Author's Guild said on Thursday, noting that sales of audiobooks surpassed one billion dollars in 2007.

"While the audio quality of the Kindle 2, judging from Amazon's promotional materials, is best described as serviceable, it's far better than the text-to-speech audio of just a few years ago," the guild said.

"We expect this software to improve rapidly," it added.

The guild warned its members to make sure that Amazon's use of the rights to their books was part of their contract with publishers.

"Until this issue is worked out, Amazon may be undermining your audio market as it exploits your e-books," the guild said.

"Bundling e-books and audio books has been discussed for a long time in the industry," it said. "It's a good idea, but it shouldn't be accomplished by fiat by an e-book distributor."

Amazon said the experimental "Read-To-Me" feature "converts words on a page to spoken word so customers have the option to read or listen.

"Customers can choose to be read to by male or female voices and can choose the speed to suit their listening preference," the company said.