The US State Department had to shut down its unclassified computer network over the weekend after evidence emerged that it could have been hacked, the US media reported late Sunday.
The State Department said in an email late Friday that the shutdown came as scheduled routine maintenance to its main unclassified network, and would impact email traffic and access to public websites.
But on Sunday reports emerged that there was evidence a hacker may have breached the security in portions of the system handling non-classified emails.
A senior official told the Washington Post there had been "activity of concern" but that none of the departments classified systems had been compromised.
If hacked, the State Department would be the latest in a series of government agencies to face cyber security breaches -- though it is not clear if there is any link between the incidents.
Last week, the US Postal Service said hackers stole sensitive personal information from its employees in a large data breach this year, and got some customer data as well.
A USPS spokesman said the breach affected as many as 800,000 people who are paid by the agency, including employees and private contractors.
The statement said hackers also penetrated payment systems at post offices and online where customers pay for services.
The agency was working with the FBI and other law enforcement in an investigation.
And last month, the White House reported an intrusion in its unclassified computer network.
In the course of addressing the breach, some White House users were temporarily disconnected from the network, an official said, but the computers and systems were not damaged.
The Washington Post quoted sources as saying hackers believed to be working for the Russian government were believed to be responsible.