US intelligence leaker Edward Snowden said he wanted to claim asylum in Russia until he can travel on to Latin America, as Washington kept up the pressure with a warning to Moscow.

In his first encounter with the outside world since becoming stranded at Moscow's Sheremetyevo airport three weeks ago, Snowden met Russian rights activists and lawyers.

He is still looking for a safe haven from US attempts to extradite him to face espionage charges for disclosing extensive American surveillance activities.

Washington warned Moscow against allowing Snowden to stay in the country and continue his embarrassing revelations.

President Barack Obama spoke by phone with Russian leader Vladimir Putin on Friday, as previously scheduled. No details were released but the White House had said Snowden would be discussed.

"Providing a propaganda platform for Mr Snowden runs counter to the Russian government's previous declarations of Russia's neutrality," White House spokesman Jay Carney said.

"It's also incompatible with Russian assurances that they do not want Mr Snowden to further damage US interests."

Carney renewed a US call on Russia to expel Snowden so that he could be returned to American soil to face trial for leaking US national security secrets.

Amateur footage aired on television showed Snowden dressed in a grey shirt and looking relaxed as he read out a statement.

"I announce today my formal acceptance of all offers of support or asylum I have been extended and all others that may be offered in the future," he told his audience, which included representatives from Human Rights Watch and Amnesty International.

"That moral decision to tell the public about spying that affects all of us has been costly, but it was the right thing to do and I have no regrets," said the 30-year-old former National Security Agency contractor.

Snowden, who has no official travel documents, said he hoped Russia would accept his renewed asylum request so he could then work out a way to travel legally to Latin America.

Although most countries to which he has applied for asylum have rejected his request, Venezuela, Bolivia and Nicaragua have all indicated they would be open to offering Snowden a safe haven.

Human Rights Watch senior researcher Tanya Lokshina, who attended Friday's meeting, said the US embassy in Moscow had asked her to pass a message to Snowden.

The message was that they did not consider him a whistleblower and he had broken the law, she said.

But Snowden, in his statement, said the US intelligence service's covert surveillance activities violated not just the US constitution but the UN declaration of human rights.

In denouncing what he saw as illegal activities, "I did what I believed right...", he added.

His statement was posted online by the anti-secrecy website WikiLeaks.

Latin American leaders defended their right to offer asylum to Snowden at a summit of the regional bloc Mercosur held in the Uruguayan capital Montevideo.

That included the right of safe passage for those granted asylum to the country offering them refuge, said a Mercosur statement.

Mercosur also condemned an incident earlier this month when several European countries denied airspace to Bolivian President Evo Morales' plane as it flew back from Moscow, forcing him to land in Vienna.

The incident happened amid rumours that Snowden was on board.

The Mercosur leaders said they would recall their ambassadors from Spain, France, Italy and Portugal for consultation in protest over the Morales incident.

The attitude of these countries was an insult not just to Bolivia but to all Mercosur states, said the statement.

In Moscow, Snowden also commented on the Morales incident.

He accused the United States of having "taken the unprecedented step of ordering military allies to ground a Latin American president's plane in search for a political refugee".

Moscow said last week that Snowden had withdrawn his application for asylum in Russia after Putin said it was conditional on not damaging US interests.

Putin's spokesman Dmitry Peskov repeated the Russian president's condition that Snowden first had to stop any "activities harming our American partners and US-Russian relations", Russian agencies reported.

But the speaker of the Russian lower house of parliament, Putin ally Sergei Naryshkin, told state television that Russia should grant Snowden asylum, describing him as a "defender of human rights".

Participants at Friday's meeting with Snowden said he had vowed not to harm the United States. But it was not clear if this meant he was prepared to halt leaks of new information about US surveillance activities.

UN rights chief Navi Pillay meanwhile weighed in on the controversy.

"Snowden's case has shown the need to protect persons disclosing information on matters that have implications for human rights, as well as the importance of ensuring respect for the right to privacy," Pillay said in a statement.