THE Pentagon is to set up a task force to investigate why a £3bn investment in methods of defeating roadside bombs in Iraq has resulted in higher American casualties.

Deaths inflicted by improvised roadside devices (IEDs) jumped from 60% to 75% of service personnel falling victim to insurgent action between March and mid-April, despite the introduction of electronic countermeasures designed to block triggering signals.

The booby-traps are also responsible for about 60% of all British casualties, including more than 20 soldiers killed in lightly-armoured Snatch Land Rovers in southern Iraq.

The Senate committee responsible for military spending has expressed concern that the mixed military-industrial anti-bomb team has become secretive and unaccountable.

The Joint IED Defeat Organisation was set up three years ago to combat the rise in booby-trap ambushes. Pentagon officials are now questioning whether expensive hi-tech countermeasures can be the answer to an essentially low-tech problem.

The vast majority of IEDs are still made of old explosives salvaged from looted mortar bombs and artillery shells.

These are often planted in "daisy-chains" using half a dozen rounds to increase the destructive blast and set off using makeshift triggering mechanisms such as garage door remote controls and mobile phone signals.

Electronic jammers used by both US and British forces rely on blocking the frequency of the detonation signal. But it has to jam the exact frequency and is far from guaranteed.

Insurgents are also becoming more adept at hiding the bombs in animal carcasses or in painted polystyrene "rocks".

Retired US Army General Montgomery Meigs, head of the Defeat organisation, said: "In the end, the most effective means of preventing IED deaths is by going after the networks that fund, make and plant them."