The Pentagon has banned the use of portable USB drives after fears that they are being used to propagate viruses.

No official statement has been released, but internal emails have shown that the Pentagon wants to recall all USB sticks distributed to employees. Warnings emerged last week of a major outbreak of malware targeting USB drives.

"For most organisations, completely banning USB Flash drives would restrict the productivity and efficiency of end users," said Jason Holloway, sales manager for Northern Europe at portable storage firm SanDisk.

"USB drives are productivity enhancing, but the risk of malware infection must be stopped with layers of security, such as hardware-based USB encryption and password protection, and virus scanning on the drive itself."

While refusing to confirm or deny the recall, Pentagon spokesman Bryan Whitman acknowledged the global spread of USB malware.

"This is not solely a department problem, this is not solely a government problem," he told Associated Press.

Virus propagation via USB stick is a throwback to the first virus techniques in which floppy discs were used to carry the code. But for an organisation like the Pentagon to take such a drastic step, the spread of the code must be wider than usual.