U.S. diplomatic headache
Washington is also telling other countries where Snowden might end up -- notably Ecuador, which says it's analyzing an asylum request from Snowden -- that they should hand him over if he lands on their soil. They note that his U.S. passport has been revoked.
"The U.S. is advising these governments that Mr. Snowden is wanted on felony charges and as such should not be allowed to proceed in any further international travel other than is necessary to return him here to the United States," Carney said.
One rumored destination is Venezuela, whose leaders have been frequently at odds with Washington in recent years.
According to the AVN state news agency, President Nicolas Maduro said Tuesday his country has not received a formal request for political asylum from Snowden. But if it did, the government would consider it on humanitarian grounds, the agency reports.
In its quest to get Snowden, the United States has limited options. CNN senior legal analyst Jeffrey Toobin said the issue now "is much more of a political and diplomatic matter than it is a legal matter."
"In an ordinary case, sure, you need a passport to get around," Toobin said. "But here, where this case is causing increasing embarrassment for the United States, governments that want the United States to be embarrassed are only too happy to waive some of the technical legal rules."
The leak controversy
Snowden has acknowledged he leaked classified documents about the NSA's surveillance programs to the Guardian newspaper in Britain and The Washington Post.
The documents revealed the existence of programs that collect records of domestic telephone calls in the United States and monitor the Internet activity of people overseas residents.
The disclosures shook the U.S. intelligence community and raised questions about whether the NSA is eroding American civil liberties.
Snowden worked as a Hawaii-based computer network administrator for Booz Allen Hamilton, an NSA contractor, before he fled to Hong Kong last month with laptops full of confidential information.
The South China Morning Post newspaper published a story Monday quoting Snowden as saying he took the job to gather evidence on U.S. surveillance programs.
He told the Guardian that he exposed the surveillance programs because they pose a threat to democracy, but administration officials said the programs are vital to preventing terrorist attacks and are overseen by all three branches of government.
Carney questioned Snowden's assertion that he acted in defense of democratic transparency, saying his argument "is belied by the protectors he has potentially chosen -- China, Russia, Ecuador."
"His failures to criticize these regimes suggests that his true motive throughout has been to injure the national security of the United States, not to advance Internet freedom and free speech," Carney told reporters.
Snowden is seeking asylum from Ecuador, Iceland and other, unspecified countries, a WikiLeaks attorney said Monday.
Ecuador has already given WikiLeaks founder Julian Assange freedom if he can find a way out of the country's embassy in London.
In his aslyum request read by Ecuadorian Foreign Minister Ricardo, Snowden compared himself to Pvt. Bradley Manning, the U.S. soldier accused of leaking classified information through WikiLeaks.
He said U.S. officials have treated Manning inhumanely by holding him in solitary confinement, and he predicted a similar "cruel and unusual" fate for himself if he falls into U.S. hands.
Snowden has come under some criticism for seeking out help from nations with questionable histories on free speech and press freedom.
For instance, The Committee to Protect Journalists has criticized Ecuadorian President Rafael Correa's government for pushing legislation that would roll back press freedoms, calling its policies increasingly repressive.
Snowden isn't looking for "political nirvana," said Glenn Greenwald, the columnist for the Guardian who broke his revelations.
"He's searching for a place where he can be safe and remain free and participate in the debate, and Ecuador seems to be the place he has chosen," Greenwald told CNN's "The Lead."