Lieutenant General Lance Smith, deputy commander of the US Central Command, said new technologies have been used with varying success to thwart improvised explosive devices, or IEDs, but there are "no silver bullets."

Senior army officials said they are spending 4.1 billion dollars through 2005 to armor nearly all Humvees and trucks in Iraq -- an issue which has brought public complaints from US troops in recent days.

And the air force chief on Tuesday said commanders are to step up the use of cargo planes to move supplies to ease the pressure on truck convoys.

Asked whether insurgent bombings have slowed military operations in Iraq, Smith said, "They are."

"They cause us to reroute vehicles. They cause us to have to employ tactics. Although the tactics have been generally successful, we have had to introduce speed and maneuverability and protection of vehicles and (it's) caused us to have to convoy," he said.

Insurgents shifted to IEDs in August and September 2003 after recognizing that they were no match for US forces in direct attacks, military officials said.

"So they had a growing understanding that where they can affect us is in the logistics part. and so they have learned, as we have, and they have moved the fight in many cases back to the rear areas," Smith said.

"There are areas where they can do that effectively, and there are areas where we find it difficult to maintain constant (control) -- like cities and the like," he said.

US forces find or neutralize about half the bombs before they go off, driving convoys fast to outrace explosions or using devices to jam remotely controlled bombs, he said.

"And that's been effective. But it's effective for a short time. The enemy is very smart and thinking," he said.

Smith said he was meeting with a joint task force that is trying to provide technological solutions to the problem.

"We've had a number of technologies that we've tried out in the theater, some more successful than others, but no silver bullet," he said. "I don't know that we'll ever find a silver bullet."

He said several hundred truckloads of supplies are being moved by air and planners are rethinking the distribution system closer to their destination, shortening the distances trucks have to travel with supplies.

The IED threat and its impact on the force -- though not new -- has drawn renewed attention since US Defense Secretary Donald Rumsfeld was asked by a soldier in Kuwait last week why soldiers were having to dig through landfill for scrap metal to protect their vehicles.

General John Jumper, the air force chief of staff, said Tuesday that 350 trucks worth of cargo a day are now moving by air in Iraq, and the goal is to increase that number to at least 1,500 truckloads a day.

Army officials said Wednesday that they have reached an agreement with Armored Holdings Inc, its exclusive supplier of armored Humvees, to step up production from 450 to 550 vehicles a month by March in order to accelerate deliveries to Iraq.

Brigadier General Jeff Sorenson, and army procurement officer, said the army plans to have nearly all Humvees in Iraq and its heavy truck fleet armored by March. By June, 32,288 Humvees and medium and heavy trucks in Iraq -- almost the entire fleet of vehicles -- are to have some level of armored protection, the officials said.

Still, the need for armor has increased at a faster pace than the army has been able to produce it.

Currently, 80 percent of the Humvees are armored. Smaller percentages of trucks have armor on them now.

Meeting the growing requirement for armored protection "is a very, very expensive proposition," said Major General Stephen Speakes, the army's director of force development.

"As you look at our forecast, both of what we've already spent and what we're immediately forecasting to spend here over the next six or eight months or so, it's several billion dollars, as you can see: 4.1 billion dollars to be specific," he said.

"And so this is an enormously expensive program but, very frankly, the communication from the secretary of defense has been real clear, which when it involves a soldier's life, we're not into the money business," he said.

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