The U.S. military has spent hundreds of millions of dollars on armored vehicles in Iraq, but insurgents are adapting their tactics and using more sophisticated and deadlier devices.
Pentagon documents obtained by ABC News say new "explosively formed projectiles," or shaped charges, pose an "extremely serious threat" to U.S. troops.
The risks from the new explosives are serious enough that commanders had briefing documents prepared so soldiers in the field would know just how deadly these devices can be and how technically advanced the insurgents have become.
The documents show how these particular shaped charges -- which were pioneered by the Lebanese Hezbollah militant group -- are constructed from a six-to-nine-inch steel pipe filled with explosives. One end of the pipe is sealed, and a curved copper or steel plate is fitted to the other end, forming a weapon that amounts to a giant bullet.
Britain has accused Iran of providing these devices to insurgents in Iraq.
The force from the explosives can send the projectile more than a mile per second, penetrating armor up to four inches thick at a range of more than 100 yards.
"Basically you are taking about a molten jet of metal coming in one side of the vehicle, going straight through everything in its path in the vehicle," said John Pike, director of GlobalSecurity.org, a Virginia-based defense, space and intelligence policy group.
Insurgents typically pack the devices in foam to camouflage them. One was painted gray to match concrete blocks on the side of the road.
As a vehicle approaches the bomb, the triggerman turns on a motion sensor in the device. When the vehicle crosses the infrared beam, the bomb goes off. Military officials say the shaped charges are responsible for the deaths of close to 50 soldiers in Marines in recent months.
They fear they may claim the lives of many more.
ABC News' Martha Raddatz filed this report for "World News Tonight."