Indian-American Rajesh "Raj" De has left his post as the top lawyer at the National Security Agency (NSA) to return to private practice as partner at the Washington law firm of Mayer Brown.

De, who left the spy agency Friday will rejoin the firm as a partner and head of a 35-lawyer practice in June, the Hill, a news site focusing on the US Congress reported.

He first made partner at Mayer Brown in 2007, before leaving for a string of government posts.

De, 41, became the agency's general counsel in May 2012, putting him right in the centre of the debate over leaks by former contractor Edward Snowden starting in 2013, the Hill said.

"It was the most challenging time in the agency's history," he was quoted as saying in an interview in The Am Law Daily.

"Everything was so secret, for so long, that there's still a lot of misinformation about what we do."

Prior to his NSA post, De worked in the Department of Justice's Office of Legal Policy and the White House.

The Harvard-trained lawyer "played a role in preserving the status quo" on NSA's controversial surveillance programmes, the Associated Press suggested.

However, De in an interview with AP "described an NSA completely at odds with the free-wheeling, all-seeing behemoth depicted by Snowden: a rule-bound, highly regulated entity that treats the private information of Americans with utmost care."

"Reasonable folks can disagree about what NSA should or shouldn't be doing," De was quoted as saying.

"But this is not a controversy over widespread abuses by a powerful intelligence apparatus, and I don't think that is by accident."

De is one of the highest-ranking Indian Americans in government. His parents, both doctors, immigrated to Philadelphia with $16 to their names, he said.