This amphibious landing exercise is the largest marine drill South Korea and the U.S. have ever jointly held.br br Operation Foal Eagle takes place annually.br br North Korea's condemned this year's event as a threatening military drill and said it would conduct its own firing exercise.br br U.S. Commanding General Paul Kennedy insists the exercises aren't politically motivated.br br (SOUNDBITE)(English) U.S. COMMANDING GENERAL PAUL KENNEDY SAYING:br br "This exercise was not designed to react to any political situation. It's not designed to send a message. This is something that we have to do to be able to inter-operate with our allies."br br On Monday South Korea's Defence Ministry said the North had fired more than 500 artillery rounds, some of which landed in its waters.br br The South said its troops fired back.