"In the course of assessing recent threats, we identified activity of concern on the unclassified Executive Office of the President network,"a White House official was quoted as saying.
"We took immediate measures to evaluate and mitigate the activity...Unfortunately, some of that resulted in the disruption of regular services to users. But people were on it and are dealing with it," the official said.
White House officials said that the intruders did not damage any of the systems and that, to date, there is no evidence the classified network was hacked.
The FBI, Secret Service and National Security Agency are all involved in the investigation, the report said.
"Hackers thought to be working for the Russian government breached the unclassified White House computer networks in recent weeks...resulting in temporary disruptions to some services while cybersecurity teams worked to contain the intrusion," the paper said.
However, White House officials are not commenting on who was behind the intrusion or how much data, if any, was taken.
"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 official said.
US officials were alerted to the breach by an ally, the paper quoted sources as saying.
Recent reports by security firms have identified cyber -­ espionage campaigns by Russian hackers thought to be working for the government.
Targets have included NATO, the Ukrainian government and US defense contractors. Russia is regarded by US officials as being in the top tier of states with cyber-capabilities.
In the case of the White House, the nature of the target is consistent with a state-sponsored campaign, the report quoted a source as saying.
The breach was discovered two to three weeks ago, it said.
Some staffers were asked to change their passwords. Intranet or VPN access was shut off for a while, but the email system, apart from some minor delays, was never down, the report said.
The Russian intelligence service was believed to have been behind a breach of the US military's classified networks, which was discovered in 2008.

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