MOSCOW - US intelligence leaker Edward Snowden on Sunday marked three weeks stuck in a Moscow airport transit lounge, as a supporter warned the fugitive possessed even more secrets that could damage the US government.

Snowden, wanted by the United States for revealing sensational details of its vast spying operations, flew into Russia from Hong Kong on June 23 and has since languished in the transit zone of the capital's Sheremetyevo airport.  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 further straining Moscow's already tense relations with Washington.

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

Glenn Greenwald told Argentina's La Nacion paper 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." Russian President Vladimir Putin said earlier this month that Snowden could claim asylum in Russia only if he stopped harming US interests.

The condition initially prompted the fugitive to withdraw his asylum application, before Snowden on Friday indicated he did still want refuge in the country.

Russia said it was still waiting Sunday for the expected asylum request, which Snowden had said was to be filed on Friday. It was not clear whether the hold-up was due to the weekend.

The head of Russia's Federal Migration Service (FMS) Konstantin Romodanovsky said Saturday that "there is for the moment no application from E. Snowden". If one was made, it would be examined "according to normal legal procedures", he added. "For the moment, we do not know anything" about an asylum application, Kremlin spokesman Dmitry Peskov told the Interfax news agency.

-- 'He can't stay at the airport indefinitely' -- The US wants the former National Security Agency (NSA) 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 harbour.

"We would urge the Russian government to afford human rights organisations the ability to do their work in Russia throughout Russia, not just at the Moscow transit lounge," White House spokesman Jay Carney said.

"Providing a propaganda platform for Mr Snowden runs counter to the Russian government's previous declarations of Russia's neutrality," he added.

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

The United States has already rebuked China 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 who was at the closed-door meeting.

"And it seemed that the only way to have his security guaranteed in Russia was to ask for asylum," she said in a statement released by HRW.

The leftist governments of Venezuela, Bolivia and Nicaragua have all 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 Friday reaffirming the right to asylum and rejecting "any attempt at pressure, harassment or criminalisation by a state or third parties".

The bloc, meeting in the Uruguayan capital Montevideo, denounced four European countries for temporarily denying airspace to a plane carrying Bolivian President Evo Morales back from Moscow earlier this month. They apparently suspected that Snowden was on board.