The US Federal Aviation Administration has claimed Amazon is not permitted under current legislation to use drone aircraft to deliver packages to its customers.
The online shopping behemoth last December revealed it was testing the use of drones for the delivery of products weighing up to 2.3kg.
CEO Jeff Bezos at the time said the service was unlikely to be available for at least four years, and noted that the company's plans were subject to new FAA rules that aren't expected to be put in place until next year.
However, in a clarification notice [pdf] published by the FAA on its interpretation of the rules for model aircraft in the FAA Modernisation and Reform Act of 2012, the FAA said the proposed Amazon service would be considered illegal.
It used the company's plans - which it did not mention by name - as an example of operations that would be banned under the regulations, which govern the use of "model aircraft" for recreational purposes.
The FAA listed the "delivery of packages to people for a fee" - under the "not hobby or recreation" section of a table depicting which types of flights would or would not be considered recreational.
"If an individual offers free shipping in association with a purchase or other offer, FAA would construe the shipping to be in furtherance of a business purpose, and thus, the operation would not fall within the statutory requirement of recreation or hobby purpose," the FAA wrote.
The notice was published as part of a wider document seeking comment on policy governing the commercial application of small drones, as the FAA prepares to publish new rules at some point next year.
A US court earlier this year found that the FAA drone regulation - which the US Congress ordered the FAA to revisit by 2015 - had not initially been implemented legally.
The court found the FAA had failed to properly consult with the public when drafting the initial legislation - a federal requirement - therefore rendering the regulation invalid.
The FAA is currently appealing the decision, and maintains that commercial operations of drones remains barred under the current legislation.
"Compliance with these rules for model aircraft operators has been required since the Act was signed on February 14, 2012, and the explanation provided today does not change that fact," the FAA said in its notice today.
"The FAA is issuing the notice to provide clear guidance to model operators on the “do’s and don’ts” of flying safely in accordance with the Act and to answer many of the questions it has received regarding the scope and application of the rules."