Four U.S. soldiers were killed in Iraq Monday by indirect fire, the U.S. military said.

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Indirect fire is a reference to rocket and mortar fire, and the U.S. military suspects hard-line Shiite militants stage such attacks.

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Many of the U.S. troops killed were soldiers operating in the Baghdad area who were struck by roadside bombs, small-arms fire and indirect fire, according to military reports.

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There were several instances of "indirect fire" in the International Zone, formerly known as the Green Zone, a U.S. Embassy spokesman said, but there were no reports of deaths or injuries.

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The ITV is also an important component of the Expeditionary Fire Support System, which allows a mobile mortar system to provide long-range indirect fire support to infantry forces as part of the Marine Corps' "Ship to Objective Maneuver" doctrine.

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