The FBI acknowledges that fake emails came from FBI email addresses. (Photo via CNN)

The US Federal Bureau of Investigation (FBI) has acknowledged that hackers compromised its email system and sent tens of thousands of spam messages.

The 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, This is an ongoing situation.

The hackers sent spam emails to 100,000 people, according to NBC News, and warned recipients of a cyberattack on their systems.

The FBI and Department of Homeland Security regularly send legitimate emails to American companies, warning them about cyber threats targeting particular industries.

This incident is the first known instance of hackers using that same system to send spam messages to a large group of people, NBC reports.

Threat-tracking organization Spamhaus Project posted an email on its Twitter account which showed a subject line of Urgent: Threat actor in systems.

The email 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.

The FBI and CISA are aware of the incident this morning involving fake emails from an @ic.fbi.gov email account, it said. This is an ongoing situation and we are not able to provide any additional information at this time. The impacted hardware was taken offline quickly upon discovery of the issue. We continue to encourage the public to be cautious of unknown senders and urge you to report suspicious activity to www.ic3.gov or www.cisa.gov.

This comes as there has been a number of high-profile breaches of US government networks in recent months.

In June, US officials warned of a growing threat of cyberattacks against American companies and government entities.

US commerce secretary Gina Raimondo said then the threat was here to stay and may even worsen, appealing for vigilance from the private sector in the face of increasingly serious cyberattacks.