The US army is losing the battle against roadside bombs in Iraq, Pentagon figures reveal.

The toll from the booby-trap devices rose from 35% of all American fatalities in January to 80% last month, despite an outlay of more than £2.5bn on countermeasures since 2003.

Now commanders are questioning the effectiveness of spending huge sums on electronic jammers, extra vehicle armour and research teams while their soldiers continue to die in ever larger numbers.

The news comes as 14 more US soldiers were reported killed in the first three days of June, including four in a single roadside bombing in Baghdad.

A total of 127 died in May, the third worst total for US forces since the 2003 invasion of Iraq. The previous most lethal months were April and November, 2004, when 135 and 137 soldiers died in large-scale offensives in Falluja.

One officer told the Herald: "The instinctive US response is always to look for a technological solution. The only things which will solve this are better intelligence targeting the guys who design and lay the bombs and winning the support of the locals to undermine the insurgents' power base."

The Pentagon, which admits that "improvised explosive devices" - IEDs - are its single biggest problem, said it intended to spend another £2bn this year to fund experimental countermeasures.

The "Defeat IEDs" organisation it set up more than a year ago to focus efforts on countermeasures says that the insurgents are using more, better-hidden and bigger bombs to ensure maximum coalition casualties.

Major-General James Simmons, the deputy US commander in Baghdad, claims that American casualties are being distorted because the insurgents are concentrating on attacking them rather than Iraqi security forces.

"The main cause of fatalities among our personnel is large and buried IEDs. That said, it is a very painful and heart-wrenching experience. The number of attacks against US forces is up and have grown more effective in recent weeks," he added.

Officers also say the insurgents are learning to stage more complex, less easily defeated ambushes.

Rescue missions by road and helicopter to bomb attack sites are themselves coming under carefully-planned attacks.

The tactics mimic those used by the IRA against British troops in Northern Ireland, where a second booby-trap was often laid after a first had gone off to catch troops sealing off the area in what became known as a "come-on" ambush.

General Simmons said insurgent groups were displaying "a greater degree of training" and had evolved into "a thinking and adaptive enemy".

In other developments yesterday, US Apache helicopters thwarted a rocket attack on Baghdad's heavily fortified Green Zone.

A US military spokesman in Baghdad, Lieutenant-Colonel Scott Bleichwehl, said helicopters opened fire after gunmen were spotted trying to launch 12 rockets at the Green Zone.

"Reports indicate four terrorists were killed and one wounded," he said.

North of the capital, a suicide car bomber attacked a police convoy in volatile Diyala province, killing 10 people in a busy market and wounding 30 others.

Gunmen at a fake checkpoint near Diyala's provincial capital of Baquba sprayed two minibuses with bullets, killing five people.