BAGHDAD, Feb 8 (Reuters) - Four U.S. Marines were killed in combat in two separate attacks in western Anbar province, the U.S. military said on Thursday. Wednesday's deaths took to at least 11 the number of U.S. servicemen killed in Iraq in the last two days. All seven crew members and passengers aboard a U.S. Marine transport helicopter were killed on Wednesday after their aircraft crashed in Anbar province. The cause of the crash is under investigation. The vast desert province of Anbar west of Baghdad is a stronghold of the Sunni Arab insurgency, and the deadliest place for U.S. troops in Iraq. ((Baghdad newsroom, editing by Giles Elgood))