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BAGHDAD, June 3 (Reuters) - U.S. Apache helicopters strafed militants preparing to fire rockets into Baghdad's Green Zone, home of Iraq's government and the U.S. embassy, and killed four gunmen, the U.S. military said on Sunday.

The airstrike targeted militants southeast of Sadr City, a stronghold of anti-American Shi'ite cleric Moqtada al-Sadr's Mehdi Army militia, late on Saturday night.

The sprawling Green Zone is the most heavily protected area in the capital, but it has come under increasing mortar and rocket attack. The attacks have also become more accurate and deadlier.

A Reuters journalist saw an air-to-ground missile fired during Saturday's operation in which jets also circled above amid the sound of heavy machinegun fire and explosions.

Police said at least two people were killed and four wounded in the airstrikes. Sadr City residents told Reuters that a car had been hit and five people killed.

A U.S. military spokesman in Baghdad, Lieutenant-Colonel Scott Bleichwehl, said gunmen had been spotted trying to launch 12 rockets at the Green Zone.

"Two of the terrorist rockets exploded as they were strafed by the U.S. helicopters. Reports indicate four terrorists were killed and one wounded," he said.

A separate statement from the military's aviation brigade said 10 rockets and a van had been destroyed.

Bleichwehl said several militants fled the scene but were tracked by "surveillance assets" to a house in Sadr City. U.S. troops detained six men in a raid on Sunday morning, he said.

"These intelligence-driven raids by the coalition are designed to go after extremists responsible for indirect fire attacks in Baghdad against innocent civilians in recent months," Bleichwehl said.

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