(Adds fourth soldier killed)

BAGHDAD, April 3 (Reuters) - Four U.S. soldiers were killed in combat in Iraq on Monday, the U.S. military said, bringing the U.S. death toll since the invasion to 3,257.

Two U.S. soldiers and a Marine were killed in separate attacks in the volatile western Anbar province, the heartland of the Sunni Arab insurgency.

One soldier died and three were wounded when a large truck bomb exploded outside a police station in the disputed northern city of Kirkuk. Two policemen and 10 civilians were also killed.

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