BAGHDAD (Reuters) - U.S. marines have killed an estimated 300 fighters loyal to a firebrand Iraqi Shi'ite cleric in fierce clashes around the holy city of Najaf in the past two days, a senior U.S. officer said on Friday. "The number of enemy casualties is 300 KIA (killed in action)," Lieutenant Colonel Gary Johnston, operations officer for the 11th Marine Expeditionary Unit, said at a military base near the city. There was no immediate independent confirmation on Friday of the toll. U.S. officials said there were indications foreign fighters had joined the Mehdi militia of radical cleric Moqtada al-Sadr in...