Hackers managed to compromised the computer systems of the US State Department at the same time as White House systems were being attacked, according to online reports.
The State Department has not publicly released a statement, but an unnamed official confirmed the attack to Reuters.
The official said that hackers had managed to disrupt nationwide websites and employee emails, and that systems had been closed off in response.
"The department recently detected activity of concern in portions of its unclassified email system. There was no compromise of any of the department's classified systems," said the official.
"The department is implementing improvements to the security of its main unclassified network during a scheduled outage of some internet-linked systems.
"This has impacted some of our unclassified email traffic and our access to public websites from our main unclassified system. We expect our systems to be up and running soon."
The attack happened a few months ago, according to the insider, and at the same time as an attack on White House systems. That incident was linked to Russia, but at least one expert suggested that this would be hard to verify.
"Generally it is nearly impossible to attribute attacks with 100 percent certainty. But if the Kremlin is in fact implicated in this attack, it could have stark ramifications for diplomatic ties between the US and Russia," said Craig Young, security researcher at Tripwire.
"The biggest question in my mind, however, is how the White House came to learn of the breach and what indicators are casting suspicion on Russia."
V3 reported last week about attacks on the National Oceanic and Atmospheric Administration and the US Postal Service, both of which were linked to Chinese hackers.
The US regularly accuses China of state-sponsored hacks and espionage, and China regularly calls them an attention-diverting smokescreen.
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