Snowden got stuck in the "Sheremetyevo"

US intelligence leaker Edward Snowden on Wednesday spent a fourth day at a Moscow airport with his onward travel plans still a mystery after Russian President Vladimir Putin rejected calls for his extradition to the United States.

The United States told Russia it has a "clear legal basis" to expel Snowden but anti-secrecy website WikiLeaks, which helped organise his flight from Hong Kong, said he risks being stuck in Russia "permanently", AFP reports.

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

In his first comments on the chase for the former contractor that has captivated world attention, Putin on Tuesday confirmed that Snowden had arrived in Moscow but said he had never left the airport's transit zone.

"He arrived as a transit passenger... He did not cross the state border," Putin said at a news conference in Finland late Tuesday. "For us, this was completely unexpected," he added.

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

Snowden who leaked revelations of massive US surveillance programmes to the media, had been expected to board a flight for Cuba on Monday, reportedly on his way to seek asylum in Ecuador.

But he never did and Putin hinted that his onward travel plans were still unknown. His US passport has been cancelled but WikiLeaks says he left Hong Kong with a refugee document supplied by Ecuador.

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.

The US urged Russia to use all means to expel Snowden, who arrived at Moscow's Sheremetyevo airport on a flight from Hong Kong on Sunday despite the US issuing a request for his arrest in China.

"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.

Hayden said that Snowden could be expelled on the basis of his travel documents and the pending charges against him. However Putin insisted that Russia could not extradite Snowden as it has no extradition agreement with the United States.

Putin said he would prefer not to deal with cases such as those of Snowden and WikiLeaks founder Julian Assange, who is holed up in Ecuador's embassy in London to avoid allegations of sexual assault in Sweden.

"It's the same as shearing a piglet: there's a lot of squealing and not much wool," he said.

But Putin dismissed speculation that 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 British activist Sarah Harrison from its legal team "is escorting him at all times".

Snowden had been expected to travel on with the state carrier Aeroflot on Monday to Havana, but never appeared on the flight. He has not been spotted in the airport, located north-west of Moscow, and is speculated to be inside a capsule hotel in the transit zone.

There is no scheduled flight from Sheremetyevo to Havana on Wednesday. The RIA Novosti quoted unidentified sources as saying that Snowden had also booked on Tuesday's flight to Havana but the reservation had been cancelled a few hours before take-off.

The Interfax news agency cited an unnamed source in Snowden's entourage claiming he is in limbo because his passport was cancelled by the US.

"Snowden's American passport is annulled, he has no other ID with him. Therefore he is obliged to stay in the Sheremetyevo transit zone, since he can neither enter Russia nor buy a ticket," the source said.

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 government 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 on Wednesday.

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.