WASHINGTON (REUTERS) - A former US National Security Agency (NSA) employee was sentenced in a federal court in Baltimore on Tuesday (Sept 25) to five-and-a-half years in prison, after pleading guilty last December to illegally taking classified information outside the spy agency, the US Justice Department said.

Nghia Hoang Pho, 68, of Ellicott City, Maryland, removed documents that contained classified national defence information and kept them at his home without authorisation, the department said in a statement.

Pho will also be required to undergo three years of supervised release after completing his prison sentence.

Pho worked in the NSA's elite hacking unit and he removed what prosecutors described as "massive troves" of highly classified documents containing top-secret national defence information between 2010 and March 2015.

A US intelligence official previously has said on the condition of anonymity that Pho was the same NSA employee who had been identified in media reports for using Kaspersky Lab antivirus software on his home computer.

Some US officials have said software from the Moscow-based company allowed Russian intelligence agencies to pilfer sensitive secrets from the United States through Pho's computer.

"As a result of his actions, Pho compromised some of our country's most closely held types of intelligence, and forced NSA to abandon important initiatives to protect itself and its operational capabilities, at great economic and operational cost, said Robert Hur, the US Attorney for the District of Maryland, in a statement.