U.S. troops in Iraq are facing new rocket-propelled bombs used by suspected Shiite militiamen, adding to the array of weapons used against the American military there, The Washington Post reported Thursday.

The powerful bombs have killed at least 21 people this year, including at least three U.S. soldiers, The Post reported.

Military officials call the rocket-propelled devices Improvised Rocket Assisted Munitions. IRAMs use 6.7-inch rockets to launch propane tanks packed with explosives and are often fired by remote control from the backs of trucks.

IRAM attacks have the potential to kill more soldiers at once than roadside bombs or conventional mortar or rocket attacks, The Post reported. Most rocket-propelled attacks have occurred in Baghdad.

The latest reported IRAM attack occurred Tuesday at a base in northeastern Baghdad used by U.S. and Iraqi soldiers.

The devices show insurgents’ continued ability to use commonly available materials and low-tech weapons to get around U.S. security measures that have cost the military billions of dollars.

"IRAM attacks could be very tragic against us," Col. William B. Hickman, commander of the 2nd Brigade Combat Team of the 101st Airborne division, which operates in northwestern Baghdad, told The Washington Post. "We take them very seriously."

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