Fugitive US intelligence leaker Edward Snowden vanished in Moscow after failing to take a flight to Cuba on which he was booked, as Washington demanded that Moscow expel him back to the United States.
Snowden, who embarrassed US President Barack Obama with his revelations of massive surveillance programmes, failed to appear on the Aeroflot flight to Havana from where he had been expected to continue to Ecuador and claim asylum.
Russia's Interfax news agency, known for its strong security contacts, confirmed that he was not on the Havana flight and quoted an informed source as saying he was likely already out of the country.
Snowden had arrived in Moscow on Sunday from Hong Kong, from where he leaked to the media details of secret cyber-espionage programmes by both US and British intelligence agencies.
WikiLeaks founder Julian Assange said Snowden was "safe'' after leaving Hong Kong with a refugee document supplied by Ecuador after the United States revoked his passport.
The White House dubbed Snowden a traitor to his country and warned both Russia and China that their relations with the US might be damaged by their refusal to extradite him.
"We expect (the Russians) to look at the options available to them to expel Mr Snowden back to the United States,'' White House spokesman Jay Carney said.
Obama meanwhile said that the United States was following all appropriate legal channels and "working with various other countries to make sure that the rule of law is observed.''
Snowden was said by Russian officials to have spent the night in a distinctly unglamorous "capsule hotel'' at Moscow's Sheremetyevo airport awaiting his onward connection.
Accompanied by WikiLeaks activist Sarah Harrison, he had been expected to take Aeroflot's 1005 GMT flight Monday from Moscow to Havana after airline sources confirmed he had checked in and had a seat allocated.
--- `He could have left on a different plane' ---
But in a dramatic sequence of events, the flight left the terminal at Moscow's Sheremetyevo airport with a pack of hopeful journalists on board and no sign of the former National Security Agency (NSA) contractor among the passengers.
An AFP correspondent on board said that the seat he had been allocated - 17A - was glaringly empty.
Just as the plane was taking off, the Interfax news agency quoted a Russian security source and an Aeroflot source as saying that Snowden was not on board the flight to Havana.
It quoted another source familiar with the matter as saying: "Snowden, most likely, has already left the Russian Federation. He could have left on a different plane.''
After the journalists learned Snowden was likely not on the plane, the doors had already been closed and there was no way out of a long and potentially fruitless 12-hour trip to Havana.
Adding to the mystery, he has not once been seen in public in the Moscow airport since Sunday's Aeroflot flight arrived from Hong Kong.
Snowden's leaks forced Obama's administration to defend US intelligence agencies' practice of gathering huge amounts of telephone and Internet data from private users around the world.
The White House warned sharply that the decision to allow Snowden to leave Hong Kong had "unquestionably'' harmed efforts to build trust in US-China relations.
"This was a deliberate choice by the (Chinese) government to release a fugitive despite a valid arrest warrant and that decision unquestionably has a negative impact on the relationship,'' Carney said.
Interfax quoted a source close to the matter as saying Russia was studying an extradition request it has received from the United States for Snowden.
However, the source said Russia does not have the right to either "detain or deport'' Snowden because he has not officially crossed the Russian border at Moscow's Sheremetyevo international airport.
--- `In a safe place' ---
Ecuadorian Foreign Minister Ricardo Patino confirmed that the Latin American country, whose embassy in London is already sheltering the wanted Assange, was considering Snowden's asylum request.
Ecuador's outspoken leftist President Rafael Correa has championed the cause of Assange and his allies, to the fury of the United States.
Asked in Hanoi about the whereabouts of Snowden, Patino replied: "I cannot give you any information on this. We are talking to the Russian government, but we do not have that information.''
On his Twitter feed, Correa added in typical style: "Hi to the country and the world... Be sure that we are analysing the Snowden case with the greatest responsibility.''
Assange said in a teleconference in London that he knew where Snowden is and "he is in a safe place and his spirits are high.''
"Due to the bellicose threats coming from the US administration we cannot go into further detail at this time,'' he added.
Snowden abandoned his high-paying job in Hawaii and went to Hong Kong on May 20 to begin issuing a series of leaks on the NSA gathering of phone call logs and Internet data, triggering concern from governments around the world.
Hong Kong, a special administrative region under Chinese rule that has maintained its own British-derived legal system, said it had informed Washington of Snowden's exit after determining that the US government request to arrest him did not fully comply with Hong Kong legal requirements.