Pentagon documents obtained by ABC News say new "explosively formed projectiles," or shaped charges, pose an "extremely serious threat" to U.S. troops.
Such devices have killed seven troops in the past two days, officials said Tuesday.
Designed by the Lebanese Hezbollah militant group, the charges are constructed from a 6- to 9-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 is essentially a giant bullet, the report said.
"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.com, a Virginia-based defense, space and intelligence policy group.
The projectiles travel more than a mile per second, penetrating armor up to 4 inches thick at a range of more than 100 yards.