North Korea conducted artillery firing drills, state-run media reported Tuesday, as the United States and South Korea prepare to kick off a joint military exercise.

North Korea has said upcoming U.S. and South Korean military drills are a "serious challenge" to peace on the Korean peninsula, warning of "negative consequences" to exercises it framed as a major threat to its security.

Observers say they’re surprised by Pyongyang’s shift in tone towards the Ulchi Freedom Shield drills, which begin on August 18.