Drones can be remotely controlled from thousands of miles away
Insurgents in Iraq have hacked into live video feeds from unmanned American drone aircraft, US media reports say.
Shia fighters are said to have used off-the-shelf software programs such as SkyGrabber to capture the footage.
The hacking was possible because the remotely flown planes have an unprotected communications link.
Obtaining such video feeds could provide insurgents with information about sites the military might be planning to target.
Mark Ward, technology correspondent, BBC News
As its name implies, SkyGrabber is a program that grabs data being broadcast by satellites - it acts as a radio for data feeds and lets people tune into different data streams as they might radio stations.
Anyone downloading via a wire only shares that net link with a few neighbours. By contrast, anyone using a satellite net connection effectively shares all the data they are getting with everyone in the area covered by a satellite.
Those other people do not see that data because their PC is only watching for what they want. However, SkyGrabber eavesdrops on all the data being downloaded over a link and turns it back into whole files.
The way that data is sent over the net makes it very easy for anyone to reconstruct files. SkyGrabber has proved popular because it has good filters that let people sort the types of files, mp3, wmv, jpg they want to get.
It also knows about many different satellites and can be re-tuned to look at other data streams - such as those coming from drones.
The downside is that SkyGrabber users only get what other people want.
The Associated Press news agency quotes a US Department of Defense official as saying the military has also found evidence of at least one instance where insurgents in Afghanistan monitored drone video.
The breach of the Pentagon surveillance system's security in Iraq is said to have come to light when footage shot by a Predator drone was found on the laptop of an apprehended insurgent.
A senior Pentagon official is quoted by the Wall Street Journal as saying that although militants were able to view the video, there was no evidence that they were able to jam electronic signals from the aircraft or take control of them.
The unnamed official said the US defence department had addressed the issue by working to encrypt all video feeds provided by drones in Iraq, Afghanistan and Pakistan.
Predator drones can fly for several hours, remotely controlled by pilots thousands of miles away. The aircraft can carry out surveillance and attack targets with on-board missiles.
Responding to the reports, a Pentagon spokesman said: "The Department of Defense constantly evaluates and seeks to improve the performance and security of our various systems and platforms.
"As we identify shortfalls, we correct them as part of a continuous process of seeking to improve capabilities and security. As a matter of policy, we don't comment on specific vulnerabilities or intelligence issues."