The American officials have said that Russia was suspected of conducting a "sophisticated cyberattack" on the Pentagon Joint Staff emails last month, which forced it to shut down its unclassified email system.

The cyberattack reportedly hit the Pentagon Joint Staff email system on 25 July, affecting nearly 4,000 defence employees and other staff. 

The attack is said to be the handiwork of Russian hackers, and a Pentagon official told Fox News "the incident bore the hallmark of a state actor".

The cyberattack was done using social media accounts, NBC News reported. 

The complexity of the hack pointed to a possible role of the Russian government, US officials have said. 

"This attack was fairly sophisticated and has the indications . . . of having come from a state actor such as Russia," an official told The Washington Post

Authorities, however, said they could not accuse Russia with '100 percent certainty'. 

The emails of the Joint Chiefs of Staff are likley to carry sensitive information as they coordinate the United States armed forces operations. 

However, officials said no confidential data was compromised. 

Russia was earlier also thought to be behind a cyberattack on unclassified email systems at the White House and the US Department of Defense this April.