White House press secretary Jay Carney said Monday that the United States was lobbying countries that had offered asylum to Edward Snowden to return the National Security Agency leaker to the United States.

"We have been having contact through diplomatic and law enforcement contacts with all the countries that might serve as transit points or final destination points," Carney said.

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Over the holiday weekend, a trio of South American countries — Bolivia, Nicaragua and Venezuela — offered asylum to Snowden, who has admitted to releasing classified information about top-secret NSA surveillance programs.

Carney said Monday that American authorities have "made very clear that he's been charged ... with felonies and as such he should not be allowed to proceed in any further travel."

He also said he still believed Russian authorities could intervene if Snowden attempts to pass through passport control.

"While we don't have an extradition treaty with Russia, we believe there is a strong legal justification for Russia to expel Mr. Snowden," Carney said.

Snowden has been at the Moscow airport for two weeks since arriving there from Hong Kong, where he originally fled.

The United States has revoked his passport and filed espionage charges against the alleged leaker, restricting his ability to leave unless he received travel documents from a sympathetic country.

Venezuelan authorities say they have not yet been in direct contact with Snowden, and it is unclear whether he plans to accept the asylum offer.

Also unclear is how Snowden would be able to travel to Venezuela.

There is a lone commercial flight from Moscow to Venezuela that connects through Havana, but travels over both European and American air space.

Last week, European officials would not allow the Bolivian presidential plane to fly through their airspace amid rumors that Snowden was aboard. That forced an unplanned landing and led to an angry rebuke from Bolivian President Evo Morales.

But the patience of authorities in Russia appears to be waning.

Alexei Pushkov, who chairs the Russian parliament's international affairs committee and is closely aligned with the Kremlin, posted to Twitter on Monday that the offer from Venezuela was "perhaps" Snowden's "last chance to receive political asylum," according to The Associated Press.

It is unclear what else the White House could do to prevent Snowden from traveling to Venezuela, if he can overcome the logistical challenges.

American authorities preemptively filed an extradition request with the country ahead of Snowden's potential travels, but Venezuelan President Nicolás Maduro said the request had already been rejected.

House Intelligence Committee Chairman Mike Rogers (R-Mich.) told CNN on Sunday he "absolutely" believed Snowden would receive the necessary paperwork to leave Russia, and said the White House should use “every legal avenue we have” to prevent his escape.

Rogers also said the U.S. should reexamine trade agreements with the three nations "to send a very clear message that we won't put up with this kind of behavior."