The White House has asked Moscow to expel fugitive whistleblower Edward Snowden to face spying charges in the US, believing he is still in Russia.
The whereabouts of the former US spy agency contractor are unclear, after he apparently failed to board a scheduled Cuba-bound Aeroflot flight in Moscow despite being expected to board.
Speaking to reporters, President Barack Obama said: "What we know is that we're following all the appropriate legal channels and working with various other countries to make sure the rule of law is observed."
A White House spokesman said it was assumed Mr Snowden was still in Russia and sharply criticised Hong Kong for allowing him leave for Moscow despite a US extradition request.
He said the affair had damaged US-China relations while appealing to Moscow to send Mr Snowden to the US to face charges.
The US accuses him of espionage after he leaked secret information to newspapers.
Mr Snowden revealed massive US global surveillance operations including phone, email and web communications under a programme known as Prism.
Mr Snowden is seeking asylum in Ecuador which is considering his request. The foreign minister compared his case to the "persecution" of US soldier Bradley Manning on trial for leaking classified files to WikiLeaks.
An Aeroflot flight attendant said Mr Snowden was not on the Cuba-bound plane, but reporters said an unidentified man boarded from a VIP van shortly before take-off.
The seat he had been expected to occupy was taken by another passenger.
American officials assume the former US National Security Agency computer expert remains in Russia.
US Secretary of State John Kerry said the US does not know Mr Snowden's intended destination.
Mr Kerry also said during a visit to India he would be deeply troubled if China and Russia had prior notice of Mr Snowden's travel plans.
Earlier, the Kremlin maintained it was unaware of any contact between Mr Snowden and the Russian authorities.
Presidential spokesman Dmitry Peskov said: "We have no such information. Overall, we have no information about him."
Ecuador's Foreign Minister Ricardo Patino said the country was guided by human rights principles in considering the asylum request and was in "respectful" contact with Russia.
Mr Patino also read a letter from Mr Snowden to Ecuador's president in which he appealed for refuge, fearing "the risk of persecution by the government of the United States and its agents".
Mr Snowden also explained in his letter that he exposed the highly classified spy programmes because the US "is intercepting the majority of communications of the world".
He also wrote he believes he will not get a fair trial in the US, after being labelled a traitor with calls for his imprisonment or execution.
Ecuador has rejected previous US efforts at cooperation, and has been helping WikiLeaks founder Julian Assange avoid prosecution by allowing him stay at its London embassy.
WikiLeaks also said it would help Mr Snowden.
The computer analyst gave documents to the Guardian and Washington Post newspapers disclosing US surveillance programmes that collect vast amounts of phone records and online data, often sweeping up information on US citizens.
The US had formally sought Mr Snowden's extradition from Hong Kong to face espionage charges but was rebuffed, with Hong Kong officials claiming that the US request did not fully comply with their laws.
The US Justice Department rejected that claim, saying its request met all requirements of the extradition treaty between the US and Hong Kong.
During conversations last week, including a phone call on Wednesday between Attorney General Eric Holder and Hong Kong Secretary for Justice Rimsky Yuen, Hong Kong officials did not raise any issues regarding sufficiency of the US request, a Justice Department representative said.
The US has been in touch through diplomatic and law enforcement channels with countries that Mr Snowden could travel through or to, reminding them that he is wanted on criminal charges.
It also reiterated its position that Mr Snowden should only be permitted to travel back to the US, a State Department official said.
Mr Snowden's US passport has been revoked.