United States intelligence leaker Edward Snowden said in a TV interview he "was trained as a spy" and had worked undercover overseas for US government agencies.

In an advance excerpt of his interview in Moscow with NBC Nightly News that aired on Tuesday in the US, Snowden rejected comments by critics that he was a low-level analyst.

Behind the mask: Edward Snowden's revelations forced the issue of court authorisation for surveillance into the open. Credit:Reuters

Snowden worked for Dell as a contractor to the US National Security Agency (NSA) from 2009 to earlier this year, then as a contractor for management consultancy firm Booz Allen Hamilton.

Previous reports have him working as for the CIA as a security guard from age 19 and as an undercover operator in overseas posts at 23.