Reports of a virus infecting computers used by pilots to control armed military drones show even protected systems may not be secure, a U.S. expert says.

Unmanned, remote-controlled aircraft have become an important tool in America's arsenal in regions of the world where the risk of casualties are high, but reports of a virus logging keystrokes of pilots controlling them from Creech Air Force Base in Nevada is a powerful reminder of cyber risks in such systems, Indiana University cybersecurity expert Fred H. Cate said.

"First it is vital to remember that defense systems are just that -- systems -- and they are integrated with many other systems, including the Internet, email, USB and disk storage, contractor networks and the like," he said in an IU release Tuesday.

"As a result no matter how well a system is protected it is impossible to isolate it entirely from the risks borne by other systems with which it interconnects."

While the military has made no public disclosure of any direct harm from the virus, Cate warns that doesn't mean the threat is not serious.

"Second, we must remember that not all cyber risks pose immediate threats," Cate said. "Some of the most pernicious risks may take the form of longer-term threats, like sharing every keystroke of drone attacks."

"The fact that the Air Force has been unable to eradicate the virus and has continued to operate the drones using compromised systems raises important issues."