The United States increased pressure on Russia to hand over former U.S. spy agency contractor Edward Snowden, who Washington believes is in Moscow waiting for news of an asylum request to Ecuador.

Snowden arrived in Moscow from Hong Kong on Sunday afternoon and was due to depart for Havana the following day, but did not use the ticket, a Moscow airport source said on Tuesday.

The source said he was travelling with Sarah Harrison. Harrison is a British legal researcher working for the anti-secrecy group, WikiLeaks.

"She [Harrison] came together with Edward Snowden from Hong-Kong on June 23 around 5 p.m.," the source said. "He had a ticket to go to Havana on the 24th, but he did not use it. She also had one, but she didn't use it either."

Snowden, charged with disclosing secret U.S. surveillance programs, left Hong Kong for Sheremetyevo airport on Sunday, starting a cat-and-mouse chase that has frayed ties between Washington and Beijing and threatens U.S.-Russia relations.

The American would need a Russian visa to leave the transit area of the airport. The 30-year-old has not been spotted by journalists camped out there.

President Vladimir Putin, who is not shy of celebrating people who challenge Washington, has ignored U.S. requests to send Snowden home but may want him to stay in the airport to avoid being implicated in helping a fugitive.

A spokesman said the Kremlin has no information on him, suggesting Moscow does not want to further alienate President Barack Obama, who has tried to "reset" ties with Russia. Putin, a former KGB spy, also favors strong measures to combat terrorism and acts seen as hostile to the state.

"It's not in Russia's interests to anger the U.S. just for the sake of angering the U.S.," Russian commentator Anton Orekh said on Ekho Moskvy radio.

The U.S. State Department said diplomats and Justice Department officials were holding discussions with Russia, suggesting they were looking for a deal to secure his return to face espionage charges.

"Given our intensified cooperation working with Russia on law enforcement matters ... we hope that the Russian government will look at all available options to return Mr Snowden back to the U.S. to face justice for the crimes with which he is charged," spokesman Patrick Ventrell told reporters on Monday.

Jay Carney, a spokesman for the White House, said it was Washington's assumption that Snowden was still in Russia.

Russian officials say they can do little to fulfill the U.S. request to expel him if he is still in the airport transit area and not on Russian territory.

En route to Ecuador

Snowden, whose exposure of the surveillance raised questions about civil liberties in the United States, flew to Moscow after being allowed to leave Hong Kong, even though Washington had asked the Chinese territory to detain him pending possible extradition.

Snowden, until recently a contractor with the U.S. National Security Agency, had been expected to fly to Havana from Moscow on Monday and eventually go on to Ecuador, according to sources at the Russian airline Aeroflot.

But he was not seen on the Aeroflot flight that landed in Havana on Monday evening. Seat 17A had reportedly been set aside for Snowden, but reporters aboard said it was occupied by another person. It was not clear whether the plane had a section in which the American could have been concealed.

Julian Assange, founder of anti-secrecy group WikiLeaks which is assisting Snowden, said on Monday Snowden had fled to Moscow en route to Ecuador and was in good health in a "safe place" but did not say where he was.

There is no direct flight from Moscow to Quito, which has said it was considering Snowden's request.

Ecuador, like Cuba and Venezuela, is a member of the ALBA bloc, an alliance of leftist governments in Latin America that pride themselves on their "anti-imperialist" credentials. The Quito government has been sheltering Assange at its London embassy for the past year.

More documents to come out?

With Snowden's whereabouts still a mystery, Obama may face prolonged embarrassment from a young American leading the world's lone superpower on a global game of hide and seek.

Obama told reporters his government was "following all the appropriate legal channels working with various other countries to make sure the rule of law is observed."

But U.S. officials said intelligence agencies were concerned that they did not know how much sensitive material Snowden had in his possession and that he may have taken more documents than initially estimated.

He could publish more documents or they could get into the hands of foreign intelligence. The Kremlin denies knowledge of any contacts between Russian officials and Snowden, despite media speculation that the security forces could be questioning him.

Carney defended the administration's attempts to bring Snowden into U.S. custody and said his choices of where to flee belied his assertion that he was focused on supporting transparency, freedom of the press and individuals' rights.

U.S. officials reserved most of their criticism for China for helping Snowden leave despite the arrest warrant. China, in turn, has expressed "grave concern" over Snowden's allegations that the United States had hacked Chinese computers.

Carney said his escape would damage U.S.-China relations and U.S. Secretary of State John Kerry said Snowden's activities could threaten the security of China and the United States.

"People may die as a consequence to what this man did," he told CNN. "It is possible that the United States would be attacked because terrorists may now know how to protect themselves in some way or another that they didn't know before."

But to his supporters, Snowden is a whistle-blowing hero who exposed the extent of U.S. surveillance activities.

A petition, initiated by his supporters and posted on the White House website said he "should be immediately issued a full, free, and absolute pardon for any crimes he has committed or may have committed related to blowing the whistle on secret NSA surveillance programs."