U.S. officials tell NBC News that Russia launched a "sophisticated cyberattack" against the Pentagon's Joint Staff unclassified email system, which has been shut down and taken offline for nearly two weeks. According to the officials, the "sophisticated cyber intrusion" occurred sometime around July 25 and affected some 4,000 military and civilian personnel who work for the Joint Chiefs of Staff.



Sources tell NBC News that it appears the cyberattack relied on some kind of automated system that rapidly gathered massive amounts of data and within a minute distributed all the information to thousands of accounts on the Internet. The officials also report the suspected Russian hackers coordinated the sophisticated cyberassault via encrypted accounts on social media.

The officials say its not clear whether the attack was sanctioned by the Russian government or conducted by individuals. But, given the scope of the attack, "It was clearly the work of a state actor," the officials say.



They stressed that no classified information was seized or compromised and that only unclassified accounts and emails were hacked.

Read More More than 21 million affected by OPM hack



Almost immediately after the cyberattack was detected, the Pentagon took the aggressive step of shutting down the entire Joint Staff unclassified email system and Internet during its investigation. The system should be back online before the end of this week.

This story is developing. Please check back for further updates.

for the latest on the markets.