For more than half a decade US and European forces have been locked in an arms race with insurgent engineers, who try to build ever more sophisticated booby-traps to defeat ever stronger armour and defensive tactics.

When American-led forces invaded Iraq five years ago, their tank squadrons were followed by supply columns of un-protected trucks and jeeps. Now, almost all their movements are in armoured vehicles or helicopters.

In response, insurgents deployed a new generation of improvised explosive devices (IEDs), ranging from huge explosive charges buried deep under the road to sophisticated, armour-piercing devices based on Iranian designs.

"Once they see that their devices or their tactics aren’t working any more, the Iraqi and Afghan insurgents change them," said a French weapons expert who works with the defence ministry, speaking on condition of anonymity. "They increase the explosive charge or alter the way they place it. The speed with which they adapt is impressive," he said.

The effects of this evolution can be seen on the ground in Iraq, where US and Iraqi army vehicles come under attack several times per day. Most blasts are ineffective, but coalition casualties are still mounting. "For every advance we make, they make another one. They’re getting better and there is nothing we can do about it," US army Sergeant Joseph Baker told AFP last year in southern Baghdad.

Currently, there are around 400 IED attacks per month targeting Western forces in their various theatres of operation in the Middle East and southwest Asia, of which around 25 involve suicide bombers, the French expert said.

The most effective variety of roadside bombs is the so-called Explosively Formed Penetrator (EFP), which uses military-grade explosives to blast a chunk of molten metal towards its target with enough force to slice through armour.

The concept was discovered by German engineers during World War II but has become associated in the Middle East with groups backed and armed by Tehran, such as Lebanon’s Hezbollah militia and some of Iraq’s Shiite factions.

EFPs first appeared in Lebanon in the 1990s and were used by Hezbollah to deadly effect against Israeli tanks during a month-long war in 2006.

The bombs arrived on the Iraqi battlefield in May 2004, and are regularly used by Shiite groups against US forces — leading to allegations from US commanders that Iran is directly sponsoring attacks. Tehran denies this accusation.

Unexploded examples of the bomb, displayed to journalists, look deceptively simple to put together. A short, wide steel or aluminium tube is packed with a C4-style explosive and capped with a concave copper disk.

When the charge explodes the copper deforms and forms a white-hot fist-sized projectile travelling at 1,500 metres per second.

But according to US officers, while the concept is simple, the skills and equipment needed to choose the right quantity of charge and machine the right size and shape of projectile bear Iranian fingerprints.

Skilled insurgents trigger the bombs with a passive infra-red device that can detect the shape of US vehicles, especially since most coalition convoys now carry jamming devices to prevent radio-controlled detonation.

In some cases strings of linked EFPs are laid along routes in order to explode simultaneously as convoys pass. The bombs are often disguised as rocks made out of plaster or fibreglass and are hard to spot.

"To build these weapons, the hardest stage is the design," the French expert said. "You need to know exactly the right dimensions, weights, angles, products and quantities." "Once that’s done, in order to mass produce them, you just need a competent metal-worker with simple machine tools," he said.

In order to defeat the threat, the tactics of the US and allied militaries have been in constant evolution.

In Iraq, the US army first sent protective kits to be fixed to its un-armoured Humvee jeeps, then started producing an armoured variety while increasing the use of helicopters and heavier Stryker armoured vehicles.

Now, it is replacing the Humvee on the battlefield with the Mine Resistant Ambush Protected vehicle — known as the MRAP — a truck-like armoured car with a V-shaped hull to deflect the force of a blast.

At the same time, secret programmes have improved the convoys’ electronic counter measures and intelligence operations have concentrated on finding and clearing mined routes.

The next stage, according to sources such as the Strategy Page website, will be the production of armour plates made out of layers of glass and ceramic that have proved tougher than steel. "But we must not hope that we’ll be able to eliminate this threat overnight. It’s impossible," the weapons expert said. "Our goal is to do as much as we can to limit the attacks’ effectiveness, through a variety of countermeasures." -AFP