How roadside bombs have become the Iraqi guerrillas' most dangerous weapon

American troops arrested nearly 50 people yesterday after six US soldiers were killed in a series of attacks north and west of Baghdad.

Three soldiers died and six were wounded when a vehicle, possibly driven by a suicide bomber, exploded at a checkpoint near a bridge over the Euphrates at Khalidiyah, west of Fallujah. Another two US soldiers were killed when a roadside bomb blew up beside a four-vehicle convoy north of Fallujah, notorious as a centre of anti-American resistance.

It is the bomb beside or under the road which has turned out to be the most dangerous weapon facing the US army in Iraq. It usually consists of heavy artillery shells detonated either by a command wire or from longer range by a remote switch such as a mobile phone or a car door opener. Combat engineers from the 82nd Airborne Division based near Fallujah had no warning before they came to Iraq last August that they would be responsible for the deadly task of searching roads for improvised bombs. "I never heard of this type of bomb until I came to Iraq," said Private Aaron Brown, a combat engineer in forward operational base Volturno outside Fallujah.

It is a measure of how unprepared the US army command was for the guerrilla war in central and northern Iraq, which started last summer as soon as the conventional war had finished, that the use of roadside bombs caught the army by surprise.

Staff Sgt Jeremy Anderson, the leader of a squad of eight men who find and render harmless the roadside bombs, was trained to deal with conventional minefields. He found the best instruction he could get was from an old US army manual on booby traps in Vietnam. The bombers often show great imagination in their attacks on the main supply routes around Fallujah. "We even found one cemented into the underside of a bridge passing over the main highway so it would explode from above," said Sgt Anderson.

Some devices are aimed not at convoys but at his men. The most ingenious so far was a solar panel, which when the dirt covering it was brushed aside by a US soldier looking for a bomb, would be exposed to the light, complete an electrical circuit and detonate the explosives.

Just once Sgt Anderson found the person at the other end of a 400 yard-long wire attached to a bomb. It turned out to be a 12-year-old boy who was taken back to his home where his father was arrested.

The main instrument used for detecting bombs is a foot-long silver-coloured stick, which is a titanium non-metallic mine probe. Sgt Anderson and his men walk along each side of a main road. "We look for wires - anything which is out of place," he said. "We identify metal in the ground." This is particularly difficult in Iraq because the sides of the roads are used as garbage dumps, making traditional metal detectors useless.

Lying on the ground outside the cement huts at Volturno - before the war a Baath party resort beside a lake - are 122mm and 155mm shells, the main explosive device used by guerrillas. The point of the shell is removed and a blaster cap put in attached by wire to a battery, usually of the size used in a motorcycle.

"It isn't exactly rocket science," said First Lt Ron Sturgeon of the same unit. "Aside from shells, almost anything can be used, such as a fire extinguisher container which will fragment when it blows up."

The advantage of a remote detonator such as a cell phone or car door opener from the point of view of the bomber is that he can stand further away. Signals from car alarms can be scrambled but cell phones are more difficult to intercept (the bomber phones the number of a second phone attached to a battery to detonate the bomb).

None of Sgt Anderson's squad knew what they were facing when they arrived in Fallujah. "When we first came out we were winging it," said Specialist Shane Thomson. One of the many surprises for the US army in Iraq is that the little titanium mine probe has turned out to be more useful than all its gigantic battle tanks.

As the attacks highlight the fragile security situation in Iraq, the US said it envisaged a significant role for the United Nations in a planned handover of power to Iraqis. The UN secretary general Kofi Annan, who is travelling in Europe, is expected to announce as early as today that he will send a mission to Iraq to study the feasibility of holding early elections, UN diplomats in New York said. Two experts are in Iraq to assess if it is safe for UN staff to return.

* The US military said it was hunting for the two-man crew of a Kiowa armed reconnaisance helicopter that crashed yesterday in the northern Iraqi city of Mosul. It was not immediately clear if the helicopter, on a search-and-rescue mission for a river patrol that had gone missing, had been shot down.