U.S. President Barack Obama and Secretary of State Hillary Clinton (L) meet with Japan's Prime Minister Yoshihiko Noda (not pictured) at the East Asia Summit in Phnom Penh, November 20, 2012. REUTERS/Jason Reed Chinese cyberspies have been reading the private emails of Obama-administration officials and "top national security and trade officials" since 2010, according to a senior administration official and a top-secret NSA document obtained by NBC.

The email espionage — codenamed "Dancing Panda" by the US before being dubbed "Legion Amethyst" — was detected in April 2010.

"The intrusion into personal emails was still active at the time of the briefing and, according to the senior official, is still going on," NBC reported.

"Dancing Panda" has successfully attacked at least 600 targets over the last five years, according to NBC.

The period overlaps with Hillary Clinton's use of a private email server for work-related correspondences while she served as secretary of state from 2009 to 2013. Neither the official nor the document identified the specific targets of the cyberspying.

Clinton's private server is under investigation by the FBI, though Clinton is not a target of the investigation.

One of Clinton's excuses for not using a government email address was that the State Department's server was often subject to security breaches.

But the administration official told NBC that the officials' government email addresses were not hacked precisely because they are more secure than private servers.

An NSA slide shows Chinese hacker units. NSA via NBC News The email correspondences of top US officials have been the target of Chinese cyberespionage since at least 2008, when spies targeted the email accounts of then-Sen. Barack Obama's and Sen. John McCain's presidential campaigns.

In 2010, NBC notes, the Chinese hacked the private email accounts of Joint Chiefs of Staff Chairman Adm. Mike Mullen and Chief of Naval Operations Adm. Gary Roughead.

Chinese hackers have breached airlines, health-insurance companies, and other government agencies to collect intelligence on US officials and their foreign contacts.

More than 21 million people had their sensitive background and security-clearance information stolen when Chinese hackers breached Office of Personnel Management (OPM) databases in early 2014. The same hackers reportedly attacked United Airlines — the main airline flying in and out of Washington, DC's Dulles Airport.

"There's no effective defense against these attacks and, as we've seen, there's also no effective deterrence," geopolitical expert Ian Bremmer told Business Insider in June.

"China isn't trying to engage in 'integrity' attacks against the US — they don't want to destroy American institutions and architecture as, after all, they're hugely invested in American economic success," he added.

President Obama and Chinese President Xi Jinping. Reuters

“The Chinese are what I would call the bullies of cyberspace: Everybody knows what they’re doing, but nobody can stop them,” Tony Lawrence, chief executive officer of VOR Technology, a Columbia, Maryland-based cybersecurity firm that works with US defense agencies, told Bloomberg.

“These state actors, their job is to gather intelligence on other nations,” he added.