A White House computer network has been breached by hackers, it has been reported.
The unclassified Executive Office of the President network was attacked, according to the Washington Post.
US authorities are reported to be investigating the breach, which was reported to officials by an ally of the US, sources said.
White House officials believe the attack was state-sponsored but are not saying what - if any - data was taken.
In a statement to the AFP news agency, the White House said "some elements of the unclassified network" had been affected.
A White House official, speaking on condition of anonymity, told the Washington Post: "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 said the attack was consistent with a state-sponsored effort and Russia is thought by the US government to be one of the most likely threats.
"On a regular basis, there are bad actors out there who are attempting to achieve intrusions into our system," a second White House official told the Washington Post.
"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 Post quoted its sources as saying that the attack was discovered two-to-three weeks ago. Some White House staff were reportedly told to change their passwords and there was some disruption to network services.
In a statement given to Agence France-Presse, a White House official said the Executive Office of the President received daily alerts concerning numerous possible cyber threats.
In the course of addressing the breach, some White House users were temporarily disconnected from the network.
"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," the official said.
The US's National Security Agency, Federal Bureau of Investigation and Security Service were reportedly investigating.
Requests for comment were referred to the Department for Homeland Security, a spokesman for which was not immediately available. A White House spokesman has not responded to the BBC's request for comment.