Story highlights North Korea threatened to "retaliate against the U.S. with tremendous muscle" if it did not cancel the military exercises

The messages come a week after South Korea resumed a similar psychological warfare campaign

South Korea says its campaign is in retaliation for landmine blasts this month

Seoul, South Korea (CNN) North Korea blared propaganda audio messages across the border into South Korea on Monday -- the same day Seoul started military exercises with the United States and other countries.

Hours before, the North had threatened to "retaliate against the U.S. with tremendous muscle" if it did not cancel the military exercises.

The exercises, spearheaded by the U.S. and South Korea, started Monday as planned, with both nations' militaries describing them as an annual effort to improve readiness and ensure stability on the Korean Peninsula.

As far as North Korea's retaliation goes, it appeared to be in the form of propaganda messages against South Korea, a major U.S. ally in the region.

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