The Pentagon (search) is not trying hard enough to defeat the makeshift roadside bombs that are the leading killer of U.S. troops in Iraq, the commander of American forces in the Middle East said Tuesday.

Pentagon statistics show that over the past two months, the homemade, easy-to-hide weapons have accounted for a significantly higher share of U.S. battle deaths. In the final 10 days of February, for example, at least 13 of the 22 battle deaths were caused by roadside bombs.

In the first two months of this year, roadside bombs accounted for 54 percent of all battle deaths. In the final four months of 2004 they accounted for 19 percent, according to Pentagon figures.

Army Gen. John Abizaid (search), the commander of U.S. Central Command, told the Senate Armed Services Committee he was satisfied that the right people, with sufficient funds, were working on the problem.

"But I'm not satisfied that we have come up with the solutions that we could if we really rolled up our sleeves and looked at it the way it needs to be looked at," Abizaid said. That statement was the most direct public challenge to the Pentagon's approach to this deadly problem.

On another subject, Abizaid said that about 3,500 insurgents took part in election day violence in Iraq on Jan. 30, an unusually precise estimate on the threat facing coalition and Iraqi forces.

Abizaid did not cite a source for that estimate.

"It was the single most important day for the insurgents to come out in force and to disrupt," he told the committee. "They threw their whole force at us, we think, and yet they were unable to disturb the elections because people wanted to vote."

The problem of roadside bombs, which the U.S. military calls improvised explosive devices (search), or IEDs, has bedeviled the Pentagon since they began appearing in the summer of 2003. Since then they have killed and maimed hundreds of U.S. and allied troops, and Abizaid said the threat had grown to the point where it required an international effort, and not just inside Iraq.

"It's an ongoing battle, and this IED threat has migrated from Iraq to Pakistan to Afghanistan, and as long as we are fighting the enemies that we're fighting in the connected manner that they are fighting the battle, we'll see it continuing to migrate," the general said.

In Iraq there is a seemingly endless supply of available explosives and they can be adapted for use against a wide variety of targets. They have proven to be a low-tech counterpoint to the U.S. military's high-powered arsenal.

Many of the most powerful IEDs are made from 155 mm artillery shells. The insurgents have found creative ways of disguising the weapons. Smaller ones are hidden inside animal carcasses or under piles of rubbish along roads traveled by U.S. military convoys and detonated from a distance.

Some have been encased in concrete to make them look like harmless cinder blocks.

In response, the U.S. military has put more armor on its vehicles, including Humvees and supply trucks, and experimented with electronic jammers and other means of detonating IEDs before they kill.

Marine Corps Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff, told the Senate Budget Committee on Tuesday that all U.S. military vehicles in Iraq will be outfitted with the best armor by summer. Until then some will rely on the less effective add-on armor, which some soldiers have dubbed "hillbilly armor" because it is an improvised solution.

"The way it's been described makes it sound like the Beverly Hillbillies which it's not," Pace said. It has provided a degree of extra protection against IEDs and small arms fire.

Even as the U.S. forces have adapted to the IED threat, the insurgents have changed tactics.

In the past two months, IEDs have tended to be larger and more powerful — designed to kill larger numbers in a single explosion. On Feb. 25, for example, an IED attack in Tarmiya killed three U.S. soldiers and wounded nine. Another on Jan. 5 killed seven soldiers and destroyed their armored infantry carrier.

Gen. Michael Hagee, commandant of the Marine Corps, told reporters last week that a fully armored Humvee recently was "ripped apart, just torn apart" by an IED made from three linked 155 mm shells.

Hagee said the Marines have developed a computer program that calculates vulnerabilities against IEDs for Marine vehicle convoys. It uses data such as the location of electronic jamming devices and the location and degree of armor protection of Marine trucks and other vehicles.

"It tells us, OK, do you have any vulnerabilities here? Should you change the arrangement of your vehicles? Should you change where your jammers are located? And if you can't do that, should you reduce the size of the convoy?" he said.