BAGHDAD, Dec 22 (Reuters) - Iraq's armed forces stormed the center of Ramadi on Tuesday, a spokesman for the counter-terrorism units said, in a drive to dislodge Islamic State militants from their remaining stronghold in a city they captured in May. The operation to recapture Ramadi, a Sunni Muslim city on the river Euphrates some 100 km (60 miles) west of Baghdad, began in early November after a months-long effort to cut off supply lines to the city, whose fall to Islamic State was a major defeat for Iraq's weak central government.