U.S. military officials call the devices improvised rocket-assisted munitions, or IRAMs. Propane tanks fired by remote control are packed with hundreds of pounds of explosives and powered by 107-mm rockets, The Washington Post reported Thursday. Rocket-propelled bombs have killed at least 21 people, including at least three U.S. soldiers, this year.

Military officials said IRAM attacks potentially can kill many people at once because they're fired at close range and create larger explosions. IRAMs have been used mostly in Baghdad

"IRAM attacks could be very tragic against us," said U.S. Army Col. William B. Hickman, commander of the 2nd Brigade Combat Team of the 101st Airborne Division, which operates in northwestern Baghdad. "We take them very seriously."

To counter the threat the rocket-propelled bombs pose, coalition forces have increased patrols around outposts, fortified buildings and offered rewards for information about networks using the weapon, the Post said.