Amazon Kindle Fire HD not cleared for sale in US

The lack of FCC approval at this stage is likely a result of Amazon's lack of experience with wireless hardware, according to Charles Golvin, a wireless analyst at Forrester Research.

Lazarus said Apple, Motorola, now owned by Google, Samsung and Microsoft have never had such issues.

"These companies have dedicated staff whose job it is to ensure FCC compliance and they do their job very well," Lazarus added.

Sprint Nextel, a leading wireless carrier, does not release phones for pre-order unless they have already been FCC approved.

"In the vast majority of cases, we will not accept a phone into our formal lab process that has not already received FCC approval," said Sprint spokeswoman Michelle Leff Mermelstein.

A spokeswoman for AT&T, which is providing the 4G data plan for the new Kindle Fire wireless tablets, declined to comment.

FCC rules bar marketing of devices that are not approved for sale by the FCC. However, there is an exception that permits advertising as long as a disclaimer is included in the promotion, according to Mitchell Lazarus, a partner at law firm Fletcher, Heald & Hildreth.

Amazon's press release announcing the devices and the product description pages on the company's website include disclaimers saying "The 4G device has not been authorised as required by the rules of the Federal Communications Commission. This device is not, and may not be, offered for sale or lease, or sold or leased, until authorisation is obtained."

"Most companies try to get the FCC work done before they announce new products, so they don't have to make these awkward disclaimers," said Lazarus.

Amazon may be waiting longer for FCC approval because the company engineered its own 4G wireless modem to be thin enough to slip inside the new Fire tablets, Forrester's Golvin said.

Bezos highlighted the new modem during his presentation on Thursday, noting it was 2.2 millimeters thick.

Most wireless devices have modems that have been used in other gadgets that have already been approved by the FCC, smoothing the process, Golvin said.

The other new Fire tablets unveiled by Amazon on Thursday only have Wi-Fi and those devices do not have the FCC disclaimer.

"That would indicate that this is related to the new 4G LTE modem," Golvin said.