The United States and South Korea started joint naval exercises today involving three US aircraft carriers in what military officials described as a clear warning to North Korea. <br/><br/>The four-day drills that began in waters off South Korea's eastern coast come as President Donald Trump continues a visit to Asia that has been dominated by discussions over the North Korean nuclear threat. South Korea's Joint Chiefs of Staff said, the battle groups of the USS Ronald Reagan, the Theodore Roosevelt and the Nimitz are in the exercise area during the drills that run until Tuesday.<br/><br/>The exercises will also involve 11 US Aegis ships and seven South Korean naval vessels, including two Aegis ships. The Aegis technology refers to missile defense. Japan is also participating in the drill. Japan announced yesterday that it would send its largest warship to join three U.S. aircraft carriers in exercises in waters near the Korean Peninsula, a display of military force meant to pressure North Korea as US. <br/><br/>