“We're following all the appropriate legal channels and working with various other countries to make sure that the rule of law is observed,” President Barack Obama told reporters.
Some experts said it was likely that Russian spy agencies were questioning Snowden on what he knows about U.S. electronic espionage against Moscow.
“If Russian special services hadn't shown interest in Snowden, they would have been utterly unprofessional,” Igor Korotchenko, a former colonel in Russia's top military command turned security analyst, said on state Rossiya 24 television.
The Kremlin has previously said Russia would be ready to consider Snowden's request for asylum.
The Interfax news agency, which has close contacts with Russian security agencies, quoted an unidentified “well-informed source” in Moscow as saying Tuesday that Snowden could be detained for a check of his papers. The report could reflect that authorities are searching for a pretext to keep Snowden in Russia.
Snowden is a former CIA employee who later was hired as a contractor for the NSA. In that job, he gained access to documents that he gave to newspapers The Guardian and The Washington Post to expose what he contends are privacy violations by an authoritarian government.
Snowden also told the South China Morning Post newspaper in Hong Kong that “the NSA does all kinds of things like hack Chinese cellphone companies to steal all of your SMS data.” He is believed to have more than 200 additional sensitive documents in laptops he is carrying.
Some observers said in addition to the sensitive data, Snowden's revelations have provided the Kremlin with propaganda arguments to counter the U.S. criticism of Russia's crackdown on opposition and civil activists under President Vladimir Putin.
“They would use Snowden to demonstrate that the U.S. government doesn't sympathize with the ideals of freedom of information, conceals key information from the public and stands ready to open criminal proceedings against those who oppose it,” Konstantin Remchukov, the editor of independent daily Nezavisimaya Gazeta, said on Ekho Moskvy radio.
Putin has accused the U.S. State Department of instigating protests in Moscow against his re-election for a third term and has taken an anti-American posture that plays well with his core support base of industrial workers and state employees.