A computer virus that hit the US drone fleet last month created a "nuisance" but no serious threat to flight operations for the unmanned aircraft, the US Air Force said Wednesday.

Confirming the virus for the first time days after it was first reported, Air Force Space Command said computers at a Nevada air base were infected with malware but played down the problem as a minor headache.

Malware on the computers at Creech Air Force base, where drone planes over Afghanistan and elsewhere are piloted remotely, was first spotted on September 15, Space Command said in a statement.

"The infected computers were part of the ground control system that supports RPA (remotely piloted aircraft) operations," it said.

"The ground system is separate from the flight control system Air Force pilots use to fly the aircraft remotely; the ability of the RPA pilots to safely fly these aircraft remained secure throughout the incident," it said.

The malware, of a type that is "found routinely on computer networks and is considered more of a nuisance than an operational threat," had been isolated, it said.

Wired magazine first reported the virus last week, saying it had spread at Creech through removable hard drives used to load map updates and transfer mission videos from one computer to another.

Drone units at other US bases around the world have now been told to stop using the removable hard drives, the magazine reported.

A spokeswoman for Space Command said that rules prohibiting openly discussing the operational status of aircraft were waived to assure the public about the state of the drone fleet.

The Air Force "felt it important to declassify portions of the information associated with this event to ensure the public understands that the detected and quarantined virus posed no threat to our operational mission and that control of our remotely piloted aircraft was never in question," Colonel Kathleen Cook said in the statement.

The American military and intelligence agencies have come to heavily rely on the robotic aircraft to track enemy forces, with the planes transforming battlefield tactics and strategy in recent years.

The US military openly acknowledges drone flights in wars in Afghanistan, Iraq and Libya but the CIA declines to publicly discuss its covert missions using drones to take out Al-Qaeda extremists in Pakistan, Yemen and elsewhere.