Washington: A "sophisticated cyber attack" by alleged Russian hackers forced the US department of defence to shut down its joint staff unclassified e-mail system, leaving thousands of workers without mails for nearly two weeks, the US media reported.

US officials who spoke on conditions of anonymity on Thursday said that the intrusion happened around July 25 and affected about 4,000 military and civilian personnel, Xinhua reported, citing NBC News.

The officials said the hackers coordinated the cyber attack on Pentagon through social media accounts, adding that they could not decide whether the Russian government was behind the attack.

In another hacking incident last year, alleged Russian hackers breached the White House's computer system and reportedly read the US President Barack Obama's unclassified e-mails.

(IANS)