State Department spokeswoman Jen Psaki, travelling with US Secretary of State John Kerry in India, confirmed that the United States has revoked Snowden's passport due to "felony arrest warrants" against the former government contractor.
"Persons wanted on felony charges, such as Mr Snowden, should not be allowed to proceed in any further international travel, other than is necessary to return him to the United States," Psaki said.
Psaki said the revocation of Snowden's passport was "routine and consistent with US regulations" in light of the charges against him.
"Such a revocation does not affect citizenship status," she said.
Snowden, who fled to Hong Kong to reveal sensational details of cyber espionage by the United States, flew to Russia on Sunday on a commercial flight, and was requesting asylum from Ecuador's government.
He did not emerge into the main terminal area, where crowds of journalists quizzed his jet-lagged and bewildered fellow passengers if they had seen the fugitive ex-agent on the flight.
Snowden was spending the night in Moscow's Sheremetyevo Airport, and was booked on an Aeroflot flight to Cuba on Monday, Russian news agencies ITAR-Tass and Interfax reported, citing unnamed airline officials.
The SU 150 flight to Havana leaves at 10:05 GMT (20:05 AEST) today.
Reports initially said he would then fly to the Venezuelan capital Caracas, but Ecuador Foreign Minister Ricardo Patino said he had asked Quito for asylum, indicating he would head there.
Aeroflot has no direct flights from Moscow to Quito, Ecuador; travellers would have to make connections in Paris, Rome or Washington DC, which could be problematic for Snowden.
Kristinn Hrafnsson, the WikiLeaks spokesman, told Britain's Sky News that Snowden would be meeting with diplomats from Ecuador in Moscow.
WikiLeaks said he is being escorted by diplomats and legal advisers from the group.