Hackers have compromised a US Federal Bureau of Investigation email system and sent tens of thousands of messages warning of a possible cyber attack, according to the agency and security specialists.

Fake emails appeared to come from a legitimate FBI email address ending in @ic.fbi.gov, the FBI said in a statement on Saturday.

Although the hardware impacted by the incident “was taken offline quickly upon discovery of the issue,” the FBI said, adding that “this is an ongoing situation.”

The hackers sent tens of thousands of emails warning of a possible cyber attack, threat-tracking organisation Spamhaus Project said on its Twitter account.

A copy of an email posted by Spamhaus on Twitter showed a subject line of “Urgent: Threat actor in systems” and appeared to end with a sign-off from the Department of Homeland Security. The FBI is part of the Department of Justice.

Both the FBI and Cybersecurity and Infrastructure Security Agency are aware of the incident, the FBI statement said.

-AAP