MOSCOW (AFP) - US intelligence leaker Edward Snowden spent a fourth day at a Moscow airport on Wednesday with his travel plans still a mystery as Ecuador warned it could take months to consider his asylum request.

The man responsible for one of the biggest intelligence security breaches in US history has not been sighted since arriving in Moscow on a flight from Hong Kong on Sunday and according to Russia is still in a transit zone at Sheremetyevo airport.

The United States has revoked his passport and is applying persistent pressure on Moscow to hand over Mr Snowden, who leaked sensational revelations of massive US surveillance programmes to the media.

Anti-secrecy website WikiLeaks, which is assisting him and claims that Mr Snowden left Hong Kong with a refugee document supplied by Ecuador, said he risks being stuck in Russia "permanently".

Ecuador, which had earlier this week raised the prospect he would make a dash for Quito after revealing that Mr Snowden had requested asylum, said it would take some time to decide the application.

The leftist Latin American state, led by populist President Rafael Correa, is already sheltering WikiLeaks founder Julian Assange at its embassy in London as he faces extradition to Sweden over allegations of sexual assault.

"It took us two months to make a decision in the case of Assange so do not expect us to make a decision sooner this time," said Foreign Minister Ricardo Patino in Malaysia.

Venezuela's President Nicolas Maduro, who by coincidence is expected in Moscow next week for an energy summit, said Caracas would also consider any asylum request from Mr Snowden.

Russia's President Vladimir Putin had on Tuesday confirmed that Mr Snowden had arrived in Moscow but said he had never left the airport's transit zone or crossed the Russian border at immigration.

"For us, this (Snowden's arrival) was completely unexpected," President Putin said in Finland.

"Mr Snowden is a free man, the sooner he selects his final destination point, the better for us and for himself."

Mr Snowden had been expected to board a plane for Cuba on Monday but he never took the flight and President Putin hinted that his onward travel plans were still unknown.

Mr Snowden's extended stay in Moscow has prompted comparisons with the Tom Hanks hit film The Terminal about a man living in an airport, while British gambling website William Hill has opened betting on his final destination.

"Cancelling Snowden's passport and bullying intermediary countries may keep Snowden permanently in Russia," WikiLeaks said in a statement on Twitter.

Maverick Russian lawmaker and head of the Liberal Democratic Party Vladimir Zhirinovsky said that unless Mr Snowden received a new passport he risked staying at Sheremetyevo indefinitely.

"Who is he without documents? He could sit for 10 years in that room (in transit). He's going to be fed. But today, he is nobody," he said quoted by RIA Novosti.

In a curious development, Spain's renowned former rights judge Baltasar Garzon, who is legal director of WikiLeaks, said without further explanation that his firm has declined to represent Mr Snowden.

The dispute risks sharpening tensions between Washington and Moscow as well as Beijing when they are struggling to overcome differences to end the conflict in Syria.

"While we do not have an extradition treaty with Russia, there is nonetheless a clear legal basis to expel Mr Snowden," National Security spokeswoman Caitlin Hayden told AFP, saying he could be expelled on the basis of travel documents and pending charges.

However, President Putin insisted that Russia could not extradite the 30-year-old former National Security Agency (NSA) technician as it has no extradition agreement with the United States.

But President Putin also dismissed speculation that Mr Snowden - a potential intelligence goldmine - was being purposely held up at the airport to be interrogated by Russian spies.

WikiLeaks also denied he was being debriefed by the Russian security services and confirmed that Briton Sarah Harrison from its legal team "is escorting him at all times".

There was no scheduled flight from Sheremetyevo to Havana on Wednesday but there is an Aeroflot service at 1005 GMT on Thursday. RIA Novosti quoted unidentified sources as saying that Mr Snowden had also booked on Tuesday's flight to Havana but the reservation had been cancelled a few hours before take-off.

Mr Snowden abandoned his high-paying intelligence contractor 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 the US request to arrest him did not fully comply with its legal requirements.

But White House spokesman Jay Carney lashed out at Beijing, saying its failure to "honour extradition obligations" had dealt a "serious setback" to efforts to build trust with new President Xi Jinping.

The United States is applying "ill-considered pressure" that will only serve to "bring Moscow and Beijing closer together", Alexei Pushkov, head of the Russian lower house of parliament's foreign affairs committee, wrote on Twitter.