BAGHDAD, Iraq — Bombs like the titanic roadside blast that killed 14 Marines last week are becoming the biggest killers of U.S. troops in Iraq, surpassing bullets, rockets and mortars, as insurgents wage an unconventional war that has boosted the American death toll beyond 1,820.

This isn't a conflict like the world wars or Vietnam, where waves of enemy ground troops backed by artillery attacked American firebases. Gone too are the intense street battles waged last year in cities like Najaf, Karbala and Fallujah, or in Nasiriyah during the 2003 invasion.

Americans still die in mortar strikes and gunfights, like the six Marine snipers killed Aug. 1 in a rebel ambush. But surprise blasts — when the road erupts without warning or an explosives-packed car disintegrates into a fireball — have become the hallmarks of the Iraq war.

Since the end of May, more than 65 percent of U.S. military deaths in Iraq have resulted from insurgent bombings, compared with nearly 23 percent in conventional combat and 12 percent in accidents, according to figures compiled by The Associated Press.

In recent weeks, rebel bombs have been responsible for 70 to 80 percent of American soldiers killed or wounded, command spokesman Lt. Col. Steven Boylan said last week.

Of the 54 American troops who died in Iraq in July, 42 were killed either by roadside bombs, car bombs or, in one case, a land mine. As of yesterday, 30 soldiers and Marines have died this month — all but nine from bombs.

These figures document an evolution in rebel tactics. Looking back to the start of the U.S.-led war in March 2003, about 32 percent of American military deaths have been from improvised explosions, suicide bombs or other such blasts — compared with about 48 percent in firefights and other combat. About 19 percent died in accidents.

The insurgent bomb strategy is frustrating for American troops, who watch their comrades die without being able to retaliate as they've been trained: with punishing return fire.

Instead, the bombs are either piloted to their target by a suicide driver or detonated remotely by an attacker who can disappear into a crowd of civilians.

"That's the insurgent strategy, this pervasive insecurity. You can't fight against an unseen enemy," said RAND Corp. counterinsurgency expert Bruce Hoffman.

Iraq has turned into a struggle that pits Americans' conventional arms against gritty rebel innovation.

As Americans have added armor, the insurgent bombs — which the U.S. military refers to as Improvised Explosive Devices or IEDs — have gotten bigger.

Guerrillas have learned in more than two years of fighting how to make their bombs invisible and far more deadly, while taking fewer casualties themselves.

In the early days of the occupation, American soldiers would find rudimentary bombs hidden in trash bins, buried along the side of roads and hidden in drink cans and even roadkill carcasses.

The U.S. military picked up on these techniques and began cleaning roadsides, chopping down trees and clearing brush. The insurgents responded by burying bombs under gravel or asphalt.

One new bomb design features a steel plate underneath that directs the blast up into a passing vehicle. Another fires a solid steel penetrator that can pierce the armor on a Bradley Fighting Vehicle, officers and analysts say.

In January, IEDs destroyed a Bradley Fighting Vehicle and an Abrams tank — two of the most heavily armored vehicles in the U.S. arsenal.

In some cases, the detonations have been so huge that American vehicles are ripped apart as thoroughly as a suicide-bomber's car. On Wednesday, a gargantuan blast from a bomb hidden by rebels who tunneled under a road outside Haditha engulfed a 25-ton troop carrier, throwing it 30 feet and killing 14 Marines and their civilian translator.

That bomb was invisible to passing troops. Some who'd heard about the investigation said there weren't even any marks on the road to offer clues that explosives lay beneath the pavement.

The bomb probably was triggered by a hidden observer, who detonated it under the second-to-last vehicle in the convoy — a packed troop carrier.

"I've lost eight buddies in a week," Army Spc. William "Shane" Parham, a sheriff's deputy from Walton County, Ga., serving in a Baghdad-based unit, told an embedded reporter for The Atlanta Journal-Constitution. "Nobody trained us to get blown up like this."

Although the number of vehicle and roadside bombings is decreasing, U.S. commanders warn they are rising in explosive power and sophistication — enough for the Pentagon to establish an IED Task Force.

The Iraq war has brought a curious reversal of roles. The insurgents have adopted advanced American warfare concepts of attacking from beyond visual range and using remote control to keep fighters out of harm's way, RAND's Hoffman said.

At the same time, insurgents blend so well into the population that American technology is ineffective, leaving U.S. troops to fight the old way, with boots on the ground.

"We're seeing the automated battlefield, but we're not the ones who are in technological control. The enemy is. They're killing us with remote-detonated IEDs," Hoffman said. "It's like they're fighting a war by technological proxy. But that's what we're supposed to be doing."

The numbers of Americans killed in Iraq remains low, compared with the tens of thousands who died in Vietnam and Korea.

But if Iraq is considered a nation-building or stability operation, the death toll is very high measured against the U.S. experience in the Dominican Republic, Panama and the Balkans, said James Dobbins, a former Bush administration envoy to Afghanistan and now a military analyst for RAND.

The number of U.S. dead is essentially under the control of U.S. commanders: Send out more patrols and more troops will get killed.

"We are largely setting the pace of battle with our patrols and raids," said the Brookings Institution's Michael O'Hanlon.