South Korea and United States forces began an annual joint military exercise Monday aimed at testing defenses against North Korea.

The two sides say the drills, known as “Ulchi Freedom Guardian,” are defensive in nature. North Korea routinely claims military exercises in the South are rehearsals for invasion of the North.

The computer-assisted exercise, set to run until August 31, involves about 56,000 South Korean forces and around 30,000 U.S. troops.

North Korean leader Kim Jong Un on Saturday visited a front-line artillery unit responsible for the North's 2010 shelling a South Korean island in 2010. He told his troops never to tolerate enemy aggression.