The United States navy has begun a drill in the Sea of Japan today, involving three aircraft carrier strike groups. The three groups led by vessels USS Ronald Reagan, Theodore Roosevelt and Nimitz are in waters east of North Korea for the 4 day drill. A senior US armed forces officer said the unusual exercise is the first in the western Pacific since 2007. <br/><br/>A US naval source said the objective is for crew members and pilots to rehearse smooth joint operations of the 3 strike groups. Vessels from Japan's Maritime Self-Defense Force and the South Korean Navy will also join bilateral drills with the United States.<br/>