The official told the broadcaster that the hack, dubbed by Washington "Legion Amethyst" after being changed from "Dancing Panda," is still ongoing.
The secret 2014 NSA briefing corroborates and expands on a 2011 announcement by the tech giant Google blaming Chinese hackers for stealing its email service passwords belonging to US government officials.
It further reveals that hackers launched over 30 "intrusion sets" from China on targets in the United States as of last year. NBC News previously reported that at least 600 corporate, private and government targets have been successfully infiltrated.
The broadcaster added that the period of exposed email access overlaps with the time when Hillary Clinton used a private server and email account for her official communications as Secretary of State in 2009-2013.
Additionally, it cites a different NSA document leak by former contractor-turned-whistleblower Edward Snowden as saying Chinese cyberspies attempted to access four top US military officials’ emails in 2010.
The targets, on whose computer hackers attempted to install malware, included former Joint Chiefs of Staff chairman Admiral Mike Mullen and chief of naval operations Admiral Gary Roughead.
The NSA declined to comment to the broadcaster on the revelations.