WASHINGTON (Reuters) - Former U.S. spy agency contractor Edward Snowden, who leaked details of massive U.S. intelligence-gathering programs, said in a U.S. TV interview he “was trained as a spy” and had worked undercover overseas for U.S. government agencies.

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

“Well, it’s no secret that the U.S. tends to get more and better intelligence out of computers nowadays than they do out of people,” Snowden told NBC news anchor Brian Williams.

“I was trained as a spy in sort of the traditional sense of the word in that I lived and worked undercover overseas - pretending to work in a job that I’m not - and even being assigned a name that was not mine.”

Describing himself as a “technical expert,” Snowden said: “I don’t work with people. I don’t recruit agents. What I do is I put systems to work for the United States. And I’ve done that at all levels from - from the bottom on the ground all the way to the top.”

He said he worked undercover overseas for both the CIA and NSA and lectured at the Joint Counterintelligence Training Academy “where I developed sources and methods for keeping our information and people secure in the most hostile and dangerous environments around the world.”

“So when they (critics) say I’m a low-level systems administrator, that I don’t know what I’m talking about, I’d say it’s somewhat misleading,” Snowden added.

Snowden, who fled to Hong Kong and then Moscow last year, is believed to have taken 1.7 million computerized documents. The leaked documents revealed massive programs run by the NSA that gathered information on emails, phone calls and Internet use by hundreds of millions of Americans.

He was charged last year in the United States with theft of government property, unauthorized communication of national defense information and willful communication of classified intelligence to an unauthorized person.

NBC is airing the full interview with Snowden on Wednesday night.