Russia reportedly launched a "sophisticated cyberattack" on the Office of the Joint Chiefs of Staff's email system, which was immediately shut down by the Pentagon following detection of the attack, and has remained down for the past 11 days, according to US officials and investigators.

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

It was not clear whether the Kremlin approved the hack, or if it was the work of independent individuals, the officials said, although the magnitude of the attack suggested that it was "clearly the work of a state actor," one of the officials said.

The Office of the Secretary of Defense did not respond to a request from VICE News for comment Thursday.

Investigators reportedly told the Daily Beast that the attack has been perpetrated over the past 11 days, starting on July 27. Officials believe the same hackers tried to breach at least five Department of Defense email accounts last month, and were also behind attacks on the White House and State Department's unclassified networks last year.

After the Daily Beast first reported the attempted Defense Department attack last month, Michael Adams, a cybersecurity expert and former US Special Operations Command employee, told the publication that hackers were employing increasingly sophisticated ways to get staff to download viruses that can then steal data from their computers.