WASHINGTON, Aug. 6 -- U.S. media reported on Thursday that Russian hackers are attacking the Pentagon's Joint Staff unclassified email system, leaving thousands of Department of Defense (DOD) workers without emails for nearly two weeks.

U.S. officials believe Moscow may have orchestrated the "sophisticated cyberattack," which infiltrated the Joint Chiefs of Staff email system sometime around July 25, according to multiple reports from NBC and CNN.

NBC reported that suspected Russian hackers initiated the "sophisticated cyber intrusion" through encrypted accounts on social media.

Media outlets quoted officials as saying that the hack on unclassified email accounts has reportedly affected around 4,000 Chiefs of Staff personnel, both military and civilian, and was conducted through an automated system that was able to siphon large amounts of information and redistribute it to thousands of web accounts.

The officials told NBC that only unclassified data was obtained, and the Pentagon immediately moved to shut down the Chiefs of Staff email system as soon as the hack was discovered. The email system will reportedly be back online by the end of the week.

They said that the digital intrusion is possibly tied to a Russian hacking group, known as APT29, which was recently profiled in a report from security firm FireEye, NBC reported.

That team uses a tactic called Hammertoss that allows hackers to clandestinely communicate with malware that has already infected a computer system, allowing it to remain undetected.

The strategy, FireEye said, reveals a "discipline and consistency" that is nearly unmatched by other top-notch hacking groups.

Meanwhile, a U.S. official was quoted by CNN saying that the methods used to crack the Pentagon's network were something government investigators had not seen previously.

DOD is working to restore email services as quickly as possible, CNN said.

So far, Russia has yet to comment on the U.S. accusations.