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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

(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.

North Korea threatens to attack U.S.

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.

It blasted the messages into South Korea using loudspeakers, the South’s Defense Ministry said, in the latest tensions between the two nations.

“It is believed that North Korea started broadcasting in order to stop South Korea’s broadcasts from reaching North Korean citizens and military in the area,” a South Korean Defense Ministry official told CNN. “North Korea’s broadcast cannot be heard clearly from the South side.”

Tit for tat

Last week, South Korea resumed a psychological warfare campaign using a similar broadcast method, the first time it has done so in over a decade.

South Korea said its campaign was in retaliation for landmine blasts on August 4 that severely injured two of its soldiers.

North Korea denies it planted the landmines on a route patrolled by South Korean troops in the demilitarized zone that separates the two nations.

Restarting the broadcasts is all but certain to infuriate North Korea, which has threatened in the past to destroy the groups of huge speakers that the South set up at the demilitarized zone that separates the two countries.

The U.S.-led United Nations Command said its investigation found that North Korea planted the landmines.

Demand for an apology

Before South Korea announced the resumption of propaganda broadcasts, Maj. Gen. Koo Hong-mo, director of operations of the South’s Joint Chiefs of Staff, warned that North Korea would “pay a harsh price” for laying the mines. He demanded that Pyongyang apologize for planting the landmines and severely punish whoever is responsible.

The demilitarized zone has divided North and South Korea since the Korean War ended in 1953 with an armistice rather than a formal peace treaty. As a result, the two countries technically remain at war.

Tension has flared in the past around sensitive points on the their de facto border, including North Korea’s shelling of an island in 2010 that killed two South Korean marines.

North Korea’s threats

In the hours leading up to the military exercises, North Korea unleashed a barrage of threats in its usual colorful language.

“The army and people of the DPRK are no longer what they used to be in the past when they had to counter the U.S. nukes with rifles,” North Korea’s National Defense Commission said.

It described the nation as “the invincible power equipped with both (the) latest offensive and defensive means unknown to the world.”

DPRK stands for the Democratic People’s Republic of Korea, the formal name for North Korea.

The multinational exercises include forces from Australia, Canada, Colombia, Denmark, France, New Zealand and the United Kingdom. They will end on August 28.

CNN’s Faith Karimi reported and wrote from Atlanta, and journalist Jungeun Kim reported from Seoul. CNN’s Chieu Luu contributed to this report.