A White House official, speaking on the condition of anonymity, said that suspicious activity has been identified on the Executive Office of the President (EOP) network.
The official, speaking to Reuters, said that mitigation efforts were ongoing, but declined to identify the exact nature of the problem.
"In the course of assessing recent threats we identified activity of concern on the unclassified EOP network. Any such activity is something that we take very seriously. In this case we took immediate measures to evaluate and mitigate the activity," the official told Reuters.
The EOP network was offline for some time last week, according to Powerline, a conservative blog.
Powerline's John Hinderaker cited unidentified sources who said that IT staffers were having problems identifying the source of the problems, but that systems were taken offline during the initial investigation. They were later restored. There also reports of delays in the press pool last Friday, as reporters received briefings and other notices later than normal last week. The delays were blamed on technical errors.
Powerline's report somewhat aligns with the White House official's statement to Reuters, which confirmed that the initial investigation resulted in temporary outages and loss of connectivity.
The investigation is focused on the unclassified network, and officials said there is no indication that classified networks were affected by the incident.
Later in the evening on Tuesday, a report from the Washington Post, again citing unnamed sources, said that the actors who breached the unclassified network were believed to be working for the Russian government.
"Certainly a variety of actors find out networks to be attractive targets and seek access to sensitive information," the official told the Post.
According to their report, U.S. officials were alerted to the intrusion two weeks ago by "an ally."
The last time the White House faced a Web-based security incident, the news was a bit more positive. In 2013, staffers successfully defended themselves against Phishing attacks from the Syrian Electronic Army.
The Phishing campaign targeted the personal email accounts of those associated with the Obama administration's social media outreach program. The SEA sent emails labeled to look like news stories from CNN and the BBC, in an attempt to collect credentials and other insider information.