The White House says it has taken steps to address suspicious activity detected on its unclassified computer network.
No details were released on who may have been responsible or when the activity occurred.
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A White House official says "activity of concern" was detected while assessing numerous possible cyberthreats that the Executive Office of the President is made aware of daily.
The situation was dealt with immediately and work continues, although the new measures have led to temporary outages and loss of connectivity for some White House employees.
The official was not authorized to discuss White House computer security by name and spoke on condition of anonymity.
The White House declined to comment on a Washington Post report that Russia is thought to be behind the breach.