A journalist revealed that Edward Snowden possessed more secrets that could damage the US government but the fugitive had chosen not to release this information.

Glenn Greenwald, the Guardian newspaper journalist who first published Snowden's revelations about the sweeping US surveillance programs said Snowden possesses data that could prove far more "damaging" to the US government.

He told Argentina's La Nacion paper Saturday that the 30-year-old had chosen not to release this information.

"Snowden has enough information to cause more damage to the US government in a minute alone than anyone else has ever had in the history of the United States," Greenwald told the paper. "But that's not his goal."

Snowden, wanted by the United States for revealing sensational details of its spying operations, flew into Russia from Hong Kong on June 23 and has been in the transit zone of Moscow's Sheremetyevo airport since.

Breaking cover for the first time since he arrived, Snowden, who has no valid travel documents, told a group of human rights activists at the airport on Friday that he was applying for asylum in Russia until he could legally travel on to Latin America.

But Russian officials have yet to confirm receiving such an application which, if approved, would risk straining Moscow's already tense relations with Washington. It was not clear whether the hold-up was due to the weekend.

The US wants the former National Security Agency contractor returned to them to face trial over the leaks.

Moscow has so far rejected that demand, saying it has no extradition treaty with Washington.

Washington has reacted sharply to the possibility that Moscow might offer Snowden a safe harbor.

US President Barack Obama spoke to Putin by telephone on Friday on issues including the Snowden affair, the Kremlin and White House both said, but no further details were forthcoming.

China has rejected US complaint for allowing Snowden to leave for Russia from Hong Kong.

At his meeting with activists, Snowden vowed he did not want to harm the US but it was not clear whether this meant he was prepared to stop leaking intelligence in order to stay in Russia.

"He said his living conditions were nothing to complain about and he was in good health - but couldn't stay at the Moscow airport indefinitely," said Tanya Lokshina of Human Rights Watch.

Three Latin American countries, Venezuela, Bolivia and Nicaragua, have offered Snowden asylum, but Snowden said Western governments could prevent him from flying to the region.

A summit of the Latin American Mercosur trade bloc issued a statement on Friday reaffirming the right to asylum and rejecting "any attempt at pressure, harassment or criminalization by a state or third parties."