The U.S. military is reinforcing the sides of its topline mine-resistant vehicles to shore up what could be weak points as troops see a spike in armor-piercing roadside bombings across Iraq, The Associated Press has learned.

The surge in attacks is putting the mine-resistant, ambush-protected vehicles (MRAPs) to the test, and so far they are largely passing. Statistics reviewed by the AP show that while bombings involving the deadly penetrating explosives have jumped by about 40 percent in the past three months, deaths in such bombings have dropped by as much as 17 percent.

Officials attribute much of the decline in deaths to the increased use of MRAPs, pronounced "M-raps." To date, about a half-dozen troops have died in incidents that involved the new bomb-resistant vehicles, and several of those deaths occurred in rollovers rather than from explosives penetrating the armor.

Military officials spoke on condition of anonymity about the statistics because some are classified. Details of specific incidents often are not provided, making it difficult to determine which type of vehicle is involved in each roadside bombing.

Army spokesman Paul Boyce said that commanders are increasing safety training to help troops better learn how to handle the heavy, ungainly vehicles.

"We're emphasizing the limitations of the vehicle's handling and the importance of understanding the lessons learned after some close calls," said Boyce, adding that the training also focuses on how to get out in an emergency. In addition, officials stress the importance of inspecting and using seat belts.

Meanwhile, at Camp Arifjahn in Kuwait, the military is reinforcing some MRAPs with additional side armor - and it shipped as many as 20 of the newly upgraded vehicles to the battlefront in April. An additional 30 are to go into Iraq beginning this month.

Navy Lt. Cmdr. James Hadley, who is overseeing the upgrades in Kuwait, said not every MRAP is getting the additional armor, which increases the vehicle's weight by as much as 5,000 pounds (2,270 kilograms). The extra protection, he said, is being added to vehicles destined for hot battleground areas.

The additional armor is shipped in kits to Kuwait and installed on the MRAPs, which only recently arrived at a facility dedicated to outfitting the vehicles with antennas and equipment before being sent to troops.

Roadside bombs have long been a primary killer of troops in Iraq, and in May 2007 Defense Secretary Robert Gates declared that the speedy purchase of MRAPs was the Pentagon's top acquisition priority. The vehicles have a V-shaped hull and sit about 36 inches off the ground, so when a bomb explodes the blast is directed out and away from the troops riding inside.

Congress has provided more than $22 billion (€14.2 billion) for at least 15,000 of the vehicles the Defense Department plans to acquire, mostly for the Army. The Marine Corps, citing reduced violence in Iraq and the awkward size of the vehicles, has already announced it wants only 2,300 - 1,400 fewer than initially planned.

The vehicles cost between $500,000 (€323,460) and $1 million (€650,000) each, depending on their size and how they are equipped.

"We will continue to improve the quality of the armor protection on the vehicles," Army Lt. Gen. Stephen Speakes said in an AP interview this week. "Our strategy will be a combination of improving the fleet that is already fielded over in combat, as well as putting additional capability on in the factory."

Speakes, deputy chief of staff for Army resources and equipping, was in Iraq recently and got to test drive one of the MRAPs and talk to commanders about their use. Roughly 3,000 of the vehicles are in Iraq now, with thousands more to be delivered in coming months.