S Korea, US launch five-day joint naval drill in face of North anger
Two separate drills began simultaneously in the East Sea (Sea of Japan) and off the southern port of Mokpo, South Korean military officials said.
The drill off Mokpo was led by the USS George Washington aircraft carrier, which will also participate in a search and rescue exercise next week with South Korean and Japanese maritime forces.
The presence of the flagship carrier has been especially galling for Pyongyang, which denounced it as a "reckless" act of provocation and a modern-day example of "gunboat diplomacy".
The joint exercises follow an unusually extended series of rocket and missile tests by North Korea, which fired 100 artillery shells into the East Sea on Monday.
South Korean and the United States hold a series of army and navy drills every year that are habitually condemned by Pyongyang as rehearsals for invasion.
Seoul and Washington insist they are defensive in nature.
The recent North Korean missile tests have coincided with various peace overtures to Seoul, including a proposal to halt all provocative military activity.
Officials from both sides are due to hold rare talks on Thursday to discuss North Korea's participation in the upcoming Asian Games in the South Korean port city of Incheon.
South Korean President Park Geun-Hye has accused Pyongyang of adopting a "two-faced attitude" by proposing a lowering of tensions while continuing its missile launches.