It has been reported that a White House computer has been hacked.
White House officials believe the attack was state-sponsored but refuse to say if any data has been taken.
A White House official said: "In the course of assessing recent threats, we identified activity of concern on the unclassified EOP network.
"Any such activity is something we take very seriously. In this case, we took immediate measures to evaluate and mitigate the activity.
"Certainly, a variety of actors find our networks to be attractive targets and seek access to sensitive information. We are still assessing the activity of concern."
The source has said the attack is consistent with a state-sponsored effort and Russia is thought by the US government to be one of the most likely threats.
A second White House official commented on the attack: "On a regular basis, there are bad actors out there who are attempting to achieve intrusions into our system.
"This is a constant battle for the government and our sensitive government computer systems, so it's always a concern for us that individuals are trying to compromise systems and get access to our networks."
The attack was discovered around two weeks ago and some White House staff were reportedly told to change their passwords as there was some disruption to the network services.
An official said: "Our computers and systems have not been damaged, though some elements of the unclassified network have been affected. The temporary outages and loss of connectivity for our users is solely the result of measures we have taken to defend our networks.
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