The Pentagon has suffered from a cyber attack so alarming that it has taken the unprecedented step of banning the use of external hardware devices, such as flash drives and DVD’s, FOX News has learned.

The attack came in the form of a global virus or worm that is spreading rapidly throughout a number of military networks.

“We have detected a global virus for which there has been alerts, and we have seen some of this on our networks,” a Pentagon official told FOX News. “We are now taking steps to mitigate the virus.”

The official could not reveal the source of the attack because that information remains classified.

“Daily there are millions of scans of the GIG, but for security reasons we don’t discuss the number of actual intrusions or attempts, or discuss specific measures commanders in the field may be taking to protect and defend our networks,” the department said in an official statement.

Military computers are often referred to as part of the Global Information Grid, or GIG, a system composed of 17 million computers, many of which house classified or sensitive information.

FOX News obtained a copy of one memo sent out last week to an Army division within the Pentagon warning of the cyber attack.

“Due to the presence of commercial malware, CDR USSTRATCOM has banned the use of removable media (thumb drives, CDRs/DVDRs, floppy disks) on all DoD networks and computers effective immediately.”

FOX