Assange was another whistle-blower who upset Washington with a massive intelligence leak. Starting from 2010, he began publishing US military and diplomatic documents on WikiLeaks with assistance from its partners in media.

Cornered in the Ecuadoran embassy in London, Assange is fighting extradition to Sweden, where he faces a sexual assault charge, which he said might result in his subsequent extradition to the US.

Amplified repercussion

Li Haidong, a researcher of US studies at the China Foreign Affairs University, said that Snowden's contact with Assange only amplified the repercussions of this incident.

"The two must somehow appreciate each other, as they are both fighting to reveal the abused government power in the West that intruded on the rights of citizens, and they are both stuck in a similar predicament," Li said.

Snowden, 29, previously divulged PRISM, a top secret NSA program that collects and analyzes data from Internet users around the world, to The Guardian and The South China Morning Post, a Hong Kong-based English-language newspaper.

Wang Xiangwei, editor-in-chief of The South China Morning Post, said on Wednesday that the newspaper still retains some "classified materials" regarding the US government that are expected to be published within one or two weeks.

According to US media reports, three former NSA employees have praised Snowden's leaks and corroborated some of his claims during a nationwide debate over the nature of his leaks.

William Binney, one of the three officials, warned about unlawful government acts if Snowden is extradited to the US, saying Snowden might be "first tortured, then maybe even rendered and tortured and then incarcerated and then tried and incarcerated or even executed".

Snowden fled to Hong Kong in May, but the US has not yet filed any formal extradition request.

US President Barack Obama on Wednesday defended the surveillance programs during his visit to Germany, saying that lives have been saved and threats avoided thanks to the monitored information.