Amazon.com apologized to buyers for having to wait up to six weeks to receive the Kindle electronic reading device after the company ran...

Amazon.com apologized to buyers for having to wait up to six weeks to receive the Kindle electronic reading device after the company ran out.

The world’s largest Internet retailer sold out of Kindles in 5 1/2 hours after they went on sale in November and has been trying to increase manufacturing capacity, Chief Executive Jeff Bezos wrote Thursday in a letter on the Seattle company’s Web site.

“We hope to be able to announce to you within the next few weeks that we’re back in stock,” Bezos wrote.

Heather Huntoon, an Amazon spokeswoman, declined to say in an e-mail how many Kindles had been sold.

The portable device sells for $399 and is about the size of a paperback.

It is able to download books, magazines and newspapers to a display screen in less than a minute. More than 100,000 books are available electronically, according to the Web site.

Amazon has boosted technology spending to add music, films and reading material that can be downloaded to reduce its reliance on sales of books, CDs and DVDs.

It also offers Unbox, which lets consumers download movies and television shows, and it started a digital-music service in September to compete with Apple’s iTunes.

The company has been expanding the content it offers for downloading. This week it completed the purchase of Audible, which sells audio versions of books and newspapers.

Amazon stock rose $3.02, or 4.3 percent, to $73.19 Thursday. The shares have declined 21 percent this year.