Washington: 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.
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 authorised 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.
Temporary outage
It was thought that hackers recently breached an unclassified computer network used by Obama's senior staff.
A White House official said Tuesday night that this prompted "countermeasures" by the administration that caused temporary system outages.
Administration officials said the attack did not appear to be aimed at destruction of either data or hardware, or to take over other systems at the White House.
That strongly suggests that the hackers' intention was either to probe and map the unclassified White House system, find entry points where they connect to other system or conduct fairly standard espionage.
That means it would be different from the kind of attack that Iran launched two years against the computer systems of Saudi Aramco and would be more in the style of the kind of attacks that Russia and China have used over the years against US government targets, a report said.
Some White House staff members lost their connections to the system "as a result of measures we have taken to defend our networks," the official said.