The State Department shut down its entire unclassified email system after a suspected hacker attack, the latest in a string of federal agencies to suffer a security breach.

A senior State Department official told Fox News on Sunday that the department recently detected "activity of concern" in its unclassified system, but its classified system was not compromised. 

On Monday, State Department Press Office Director Jeff Rathke also stressed that "no classified systems have been affected." He said the origin of the attack remains under investigation, but that they "believe that this activity was linked to the incidents" that recently targeted the White House computer network. 

Those incidents were made public in late October, but there was no indication then that the State Department had been affected. Since then, a number of agencies, including the U.S. Postal Service and the National Weather Service, have reported attacks.

On Monday, an FBI spokesman said they are aware of the State Department breach and are working to investigate. 

"The compromise of public and private sector systems is something we take very seriously, and the FBI will continue to investigate and hold accountable those who pose a threat in cyberspace," the spokesman said. 

The senior State Department official said the department shut down its worldwide email late on Friday as part of a scheduled outage of some of its Internet-linked systems to make security improvements to its main unclassified computer network. 

The official said the department expects that all of its systems will be operating as normal in the near future, but would not discuss who might be responsible for the breach. Earlier attacks have been blamed on Russian or Chinese hackers, although their origin has never been publicly confirmed. 

The latest incident has raised concern on Capitol Hill. Rep. Michael McCaul, R-Texas, chairman of the House Homeland Security Committee, said Monday that "criminals, hacktivists and nation states are attacking our government networks at an alarming rate." 

The Associated Press contributed to this report.