WASHINGTON (AP) -- The Pentagon has banned, at least temporarily, the use of external computer flash drives because of a virus threat officials detected on Defense Department networks.

While defense officials would not publicly confirm the ban, messages were sent to department employees informing them of the new restrictions. As part of the ban, the Pentagon was collecting any of the small flash drives that were purchased or provided by the department to workers, according to one message distributed to employees.

Workers are being told there is no guarantee they will ever get the devices back and it is not clear how long the ban will last.

Pentagon spokesman Bryan Whitman would provide no details on the virus Friday, but he described it as a "global virus" that has been the subject of public alerts.

"This is not solely a department problem, this is not solely a government problem," Whitman said.

The Pentagon has acknowledged that its vast computer network is scanned or probed by outsiders millions of times each day. Last year a cyber attack forced the Defense Department to take up to 1,500 computers off line.

Officials said then that a penetration of the system was detected, but the attack had no adverse impact on department operations.

However, military leaders have consistently warned of potential threats from a variety of sources including other countries -- such as China -- along with other self-styled cyber-vigilantes and terrorists.

The issue has also been of concern at the Department of Homeland Security. A September audit by the DHS Inspector General recommended that the agency implement greater procedures to ensure that only authorized computer flash drives or other storage devices can be connected to the network there and that an inventory of those devices be set up.

DHS agreed with the recommendations and said some of that is already being done. DHS also said more software enhancements are in the works that will provide more protection.