The U.S. State Department has shut down its unclassified email system due to a possible breach from a suspected hacking attack, the Tech Times reports.

Department officials said on Friday they were taking their email system temporarily offline after they detected "suspicious activity". A department official, who chose to remain anonymous, said the State Department email system was subjected to a hacking attack at the same time as when the White House computer networks were hacked several weeks ago.

"The department recently detected activity of concern in portions of its unclassified email system. There was no compromise of any of the department's classified systems," said a Department official in a statement, according to NextGov.

Taking the system offline has disrupted email communications between State Department employees, as well as delaying access to public websites. It is not certain why the breached email system wasn't taken offline much earlier than this weekend, but it is believed that government-backed hackers from a foreign nation were the primary malefactors of the security breach.

The hacking attack on the State Department, however, was not successful in infiltrating their classified systems, reports the NY Times.

This attack, however, marked the fourth time recently that a government agency became the subject of a security breach. Aside from the State Department, there have been similar events at the U.S. Postal Service, as well as the National Oceanic and Atmospheric Administration.

The attack on the systems of the U.S. Postal Service successfully managed to compromise the agency database containing personal contact information of customers who called the agency via call center between January and August this year.

A senior U.S. official said their unclassified email systems are expected to be back online shortly, according to Reuters.

"The department is implementing improvements to the security of its main unclassified network during a scheduled outage of some internet linked systems," said the official. "This has impacted some of our unclassified email traffic and our access to public websites from our main unclassified system. We expect our systems to be up and running soon."