WASHINGTON/MOSCOW?- The United States on Monday increased pressure on Russia to hand over Edward Snowden, the American charged with disclosing secret U.S. surveillance programs, and said it believed he was still in Moscow despite reports he was leaving for Cuba.

Earlier Snowden, until recently a contractor with the U.S. National Security Agency, had been expected to fly to Havana from Moscow, perhaps on the way to Ecuador, but he was not seen on the plane and Russian officials declined to say where he was.

The U.S. State Department said diplomats and Justice Department officials were engaged in discussions with Russia, suggesting they were looking for a deal to secure his return. ?

"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. ?

Snowden flew to Moscow after being allowed to leave Hong Kong on Sunday, even though Washington had asked the Chinese territory to detain him pending his possible extradition on espionage charges.

White House spokesman Jay Carney defended the administration's attempts to bring Snowden into U.S. custody and blamed China for assisting in his departure from Hong Kong. He said it would damage U.S.- China relations.

Sources at the Russian airline Aeroflot had said on Sunday that Snowden would be on a flight on Monday morning that arrived in Havana at 6.45 p.m./2245 GMT, but reporters who took the flight said another person occupied seat 17A, which had been set aside for him. ?

"He didn't take the flight (to Havana)," a source at Aeroflot told Reuters.

However, before the plane left for Cuba, a white van for VIPs approached it on the tarmac. Police stood by as a single man in a white shirt climbed the stairs on to the plane soon afterwards but he could not be identified by reporters watching in the transit area. It was not clear whether the plane had a section in which Snowden could have been concealed.

Julian Assange, the founder of anti-secrecy group WikiLeaks which is assisting Snowden, said the 30-year-old had fled to Moscow en route to Ecuador and was in good health in a "safe place" but did not say where he was now.

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.

Washington was stung by the defiance from Russia, with which President Barack Obama has sought improved relations.

One of three high-powered lawyers representing Snowden in Hong Kong said they had warned him he might be stuck in legal limbo for years - and possibly detained - if he stayed put and requested asylum in the city.

Carney, speaking several hours after the Moscow-Havana flight took off, said it was the U.S. assumption that Snowden was still in Russia and pressed Russia to use all options to expel him to the United States.

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

U.S.-RUSSIA DISCUSSIONS

Carney slammed those countries from which Snowden had chosen to seek protection, saying his choice belied his claim that he was focused on supporting transparency, freedom of the press and individuals' rights.

He sharply criticised Hong Kong. "This was a deliberate choice by the government to release a fugitive despite a valid arrest warrant, and that decision unquestionably has a negative impact on the U.S.-China relationship," he said.

U.S. Secretary of State John Kerry told CNN Snowden's activities could threaten the security of China as well as that of the United States. ?

"People may die as a consequence to what this man did," he said. "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. This is a very dangerous act."

To his supporters, however, Snowden is a whistleblowing hero who exposed the extent of U.S. surveillance activities.

A petition initiated by his supporters and posted on the White House website described him as "a national hero and 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."

The petition had garnered more than 113,000 signatures by 2030 GMT, above the 100,000 needed to oblige a White House response within 30 days.

RUSSIA SEES U.S. HYPOCRISY

Russian President Vladimir Putin's press secretary denied any knowledge of Snowden's movements. Asked if Snowden had spoken to the Russian authorities, Dmitry Peskov said: "Overall, we have no information about him."

Other Russian officials said Moscow had no obligation to cooperate with Washington, citing legislation passed in the United States to impose visa bans and asset freezes on Russians accused of violating human rights.

The Russian news agency Interfax quoted an unnamed source as saying Moscow could not arrest or deport Snowden because he had not actually entered Russian territory - suggesting he had remained in the transit area at Moscow's Sheremetyevo airport.

Putin has missed few chances to champion public figures who challenge Western governments and to portray Washington as an overzealous global policeman.

WikiLeaks said Snowden was supplied with a refugee document of passage by Ecuador and that a British legal researcher working for the anti-secrecy group had accompanied him.

Ecuador's foreign minister, Ricardo Patino, said during a trip to Vietnam that Quito would take into account a U.S. request about Snowden and was in contact with Russia about him. He gave no details of the U.S. request.

Snowden, who worked as a systems administrator at a U.S. National Security Agency facility in Hawaii for about three months, had been hiding in Hong Kong, since leaking details about secret U.S. surveillance programs to news media.

He said in an interview published by Hong Kong's South China Morning Post on Monday that he took a job at U.S. contractor Booz Allen Hamilton deliberately to gain access to details of the NSA's surveillance programs.

"My position with Booz Allen Hamilton granted me access to lists of machines all over the world the NSA hacked," Snowden said, according to the article.

Booz Allen Hamilton fired Snowden on June 10.

U.S. officials said intelligence agencies were worried they do not know how much sensitive material Snowden had in his possession and he may have taken more documents than initially estimated. They were concerned that his links with WikiLeaks would increase the likelihood of their being published.

Snowden has been charged with theft of federal government property, unauthorised communication of national defence information and wilful communication of classified communications intelligence to an unauthorised person. The last two charges fall under the U.S. Espionage Act