The explosion that killed 14 marines in Haditha yesterday was powerful enough to flip the 25-ton amphibious assault vehicle they were riding in, in keeping with an increasingly deadly trend, American military officers say.



In recent months the roadside bombs favored by insurgents in Iraq have grown significantly in size and sophistication, the officers say, adding to their deadliness and defeating efforts to increase troops'' safety by adding armor to vehicles.



The new problems facing the military were displayed more than a week earlier, on July 23, when a huge bomb buried on a road southwest of Baghdad Airport detonated an hour before dark underneath a Humvee carrying four American soldiers.



The explosive device was constructed from a bomb weighing 500 pounds or more that was meant to be dropped from an aircraft, according to military explosives experts, and was probably Russian in origin.



And what happened in the aftermath of the July 23 attack provided further cause for alarm.



A British explosives expert, part of a special squad formed to investigate major insurgent bomb attacks, stepped on a second, smaller bomb buried near the first and was badly wounded, two American officers said. He later had an arm and a leg amputated. A third device, hidden a few yards away, was found and defused.



Military personnel involved said the attack last month indicated to them that a new and deadly bomb-making cell singling out American patrols was operating near the large allied military base at the airport, an area that two officers said had seen little insurgent activity in months.



There was further evidence for that on Saturday. Less than a mile from the July 23 attack, four more American soldiers were killed when their Humvee was struck by another hidden bomb.



In addition to the recent attacks in Haditha and near the airport, 10 marines were killed in two separate incidents in western Iraq in June when their armored Humvees were destroyed by roadside bombs, officials said.



From the earliest days of the insurgency there has been a constantly evolving battle of wits between insurgent bombers and soldiers trying to stop the roadside bombs and suicide attacks.



As the threat from bombs and suicide attacks has grown, the Pentagon has rushed 24,000 armored Humvees to Iraq since late 2003. But the insurgents have responded by building bombs powerful enough to penetrate the vehicles'' steel plating.



Senior American commanders say they have also seen evidence that insurgents are making increased use of "shaped" charges, which concentrate the blast and give it a better chance of penetrating armored vehicles, causing higher casualties.



Bomb-making techniques used by the anti-Israeli militant group Hezbollah in Lebanon have increasingly begun appearing in roadside bombs in Iraq. A senior American commander said bombs using shaped charges closely matched the bombs that Hezbollah used against Israel.



"Our assessment is that they are probably going off to school" to learn how to make bombs that can destroy armored vehicles, the officer said.



As the military has begun conducting post-bombing investigations, insurgents have increasingly been planting multiple devices at the same location, apparently to disrupt investigative teams sent to the blast site, or at least delay their work while they clear the site of any secondary bombs.



Sometimes improvised explosive devices, known as I.E.D.''s, are placed in the open to draw in American disposal units. "A lot of times they plant fake I.E.D.''s and wait until you come on site to open up," said Sgt. Burnell Zachary. "Once the mortar rounds stop, the drive-bys come."



Americans directly engaged in the fight say that while they are having some success at tracking down some of the perpetrators, there is a steady supply of Iraqis willing to set bombs for a small amount of money.



At best, American soldiers familiar with the bomb problem say, they may be able to reduce the number of attacks, which average around 65 a day against Iraqis and Americans troops, and hand over the fight to Iraqi security forces sometime next year.



"It''s not realistic to think we will stop this," says Sgt. Daniel McDonnell, who leads a three-man team of explosives technicians responsible for finding and defusing improvised explosive devices in Baghdad. "We''re fighting an enemy that goes home at night and doesn''t wear uniforms. But we can get it to an acceptable level."



(Edited)



NYT: Insurgents using bigger, more lethal bombs, U.S. officers say