Edward Snowden and reporter Glenn Greenwald appeared together via video link on Saturday warning of wider US spy operations.

Edward Snowden and reporter Glenn Greenwald, who brought to light the whistleblower's leaks about mass U.S. government surveillance last year, appeared together via video link on Saturday warning of wider US spy operations.

The pair cautioned that government monitoring of "metadata" is more intrusive than directly listening to phone calls or reading emails and stressed the importance of a free press willing to scrutinize government activity.

Metadata includes which telephone number calls which other numbers, when the calls were made and how long they lasted. Metadata does not include the content of the calls.

Amnesty International is campaigning to end mass surveillance by the U.S. government and calling for Congressional action to further rein in the collection of information about telephone calls and other communications.

Last year, Snowden, who had been working at a NSA facility as an employee of Booz Allen Hamilton, leaked a raft of secret documents that revealed a vast U.S. government system for monitoring phone and Internet data.

Snowden faces arrest if he steps foot on U.S. soil.

U.S. President Barack Obama said last month he plans to ask Congress to end the bulk collection and storage of phone records by the NSA but allow the government to access metadata when needed.

Snowden and Greenwald said that such data is in fact more revealing than outright government spying on phone conversations and emails.

"Metadata is what allows an actual enumerated understanding, a precise record of all the private activities in all of our lives. It shows our associations, our political affiliations and our actual activities," said Snowden.

Greenwald, who met with Snowden 10 months ago and wrote about the leaked documents in the Guardian and other media outlets, promised further revelations of government abuses of power at his new media venture the Intercept.

"My hope and my belief is that as we do more of that reporting and as people see the scope of the abuse as opposed to just the scope of the surveillance they will start to care more," he said.

"Mark my words. Put stars by it and in two months or so come back and tell me if I didn't make good on my word."