Fighters in Iraq and Afghanistan have been able to use low cost software downloaded from the internet to hack into live video feeds from unmanned US surveillance aircraft, a military official has said.

Speaking on condition of anonymity, a senior US defence official told the Associated Press the fighters could view feeds from Predator drones - the US military's eyes in the sky for intelligence gathering - but could not take control of the aircraft or jam its electronic signals.

The fighters reportedly used software that costs less than $30 to hack into the video feed from the drones.

The Wall Street Journal first reported on Thursday that Shia fighters in Iraq had used software programs such as SkyGrabber, available for as little as $25.95 on the internet, to regularly capture drone video feeds.

According to the paper the issue came to light when the US military in Iraq apprehended a fighter whose laptop contained files of intercepted drone video feeds.

In recent months the US military has also reportedly found evidence of at least one instance where fighters in Afghanistan also monitored US drone video feeds.

Encryption

The interception is possible because communications links with the remotely flown planes are unprotected.

Officials said when the intercepts were discovered in July last year, the Pentagon began a programme to start encrypting all of its drone video feeds from Iraq, Afghanistan and Pakistan.

However, there are at least 600 unmanned vehicles along with thousands of ground stations to address and the upgrade is expected to take some time to complete.

Dale Meyerrose, a former chief information officer for the US intelligence community, compared the problem to street criminals listening to police scanners.

"This was just one of the signals, a broadcast signal, and there was no hacking," he told the Associated Press.

The US Predator drone, also currently used in Pakistan, Somalia and elsewhere, can fly for hours remotely controlled by pilots thousands of kilometres away.

It can fly armed or unarmed, and is part of a growing arsenal of such aircraft that includes the Reaper and Raven as well as a new, high-tech video sensor system called the Gorgon Stare, being installed on Reapers.