Icann oversees much of the basic functionality of the net
Plans to offer hundreds of new web addresses as alternatives to .com have been criticised by the US government.
The Internet Corporation for Assigned Names and Numbers, which oversees net addresses has floated plans for the radical change to the existing system.
But the US Commerce department has questioned both the benefits and the costs of such a scheme.
Officials have also raised concerns about whether the plans will destabilise the current system.
The US government's response includes two letters sent separately by officials raising concerns about the plans.
In a letter sent last week, Commerce Department official Meredith Baker questioned Icann's proposed pricings for the new addresses.
The net body planned to offer between 200 and 800 new addresses for sale at $185,000 (£125,000) per domain with a yearly cost of $60,000.
But Ms Baker said Icann needed to "articulate a clear rationale for the proposed fee structure".
She also called on the net body to supply evidence that the changes would "not compromise the stability or security of DNS (Domain Name System)".
There are currently 21 so-called gTLDs (Generic Top Level Domains). They include .com, .org, .net and .gov.
In June 2008 Icann announced proposal to allow an unlimited number of new gTLDs, citing greater competition and lower costs as the benefits of a more open system.
But another Department of Commerce official, Deborah Garza, has questioned the plan.
In her own letter to Icann she wrote: "The DOC is unconvinced that new gTLDs will alter the preference for .com domains, and fears that a huge number of new gTLDs will simply force companies to register them in order to maintain and redirect appropriate traffic".
The criticisms are pertinent because the US government is still nominally in charge of Icann despite calls from the net body for administrative freedom.