Less than two weeks after North Korea completed its second successful nuclear warhead test of the year, South Korea and the United States are reportedly training for a “mock attack” on a nuclear facility, CNN reported Wednesday morning.

The South Korean Defense Department told CNN the plans aren’t direct preparations for North Korea and that it will also work on a mock “sudden missile attack.” The drills will take place in Alaska between Oct. 3 and Oct. 21 and will center around the use of a GBU-31 Joint Direct Attack Munition, a type of system that switches a bomb from unguided to guided.

The announcement comes on the heels of two demonstrations of military unity between the U.S. and South Korea against the Kim Jong Un regime. Two supersonic U.S. bombers flew over South Korea Wednesday and one landed just south of the capital Seoul, eight days after South Korean fighter jets escorted two U.S. B-1 bombers the closest they have ever been to North Korea, Reuters reported.

"Today marks the first time the airframe has landed on the Korean peninsula in 20 years, as well as conducting the closest flight near North Korea ever," the U.S. Air Force posted on its website.

North Korea has conducted five nuclear warhead tests this year, with the last coming on Sept. 9. It announced a ground test of a long-range rocket engine Tuesday that could be used to launch a satellite into space. Critics, however, believe is a cover for a nuclear weapons program, the New York Times reported.

North Korea also claimed to have successfully tested a submarine-based ballistic missile on Aug. 24, which Kim dubbed “the greatest success and victory.”