Trash-laden North Korean balloons were found on the grounds of South Korea’s presidential office compound, officials said Wednesday, the latest in a series of incidents that have raised tensions and rhetoric on the Korean Peninsula.

More than 3,000 North Korean balloons, often filled with garbage such as cigarette butts, discarded batteries and even manure have fallen in the South since May, according to South Korean officials – who have responded by resuming loudspeaker broadcasts of propaganda and entertainment like K-pop songs along the demilitarized zone (DMZ).

The discovery at the presidential office Wednesday came after South Korean authorities warned the public to beware of falling objects as suspected North Korean trash balloons moved south toward the northern area of Gyeonggi province.

“While monitoring trash balloons sent by North Korea in cooperation with the Joint Chiefs of Staff today, we identified trash that fell in the presidential office area in Yongsan,” the presidential security service said later in a statement.

“No harmful or contaminating substances were found from an analysis by the response team.”

South Korea’s Joint Chiefs of Staff (JCS) has advised people not to touch fallen balloons and to report any found to authorities.

“North Korea’s actions clearly violate international law and seriously threaten the safety of our citizens,” JCS said in a statement after an earlier balloon incident. “All responsibility arising from the North Korean balloons lies entirely with North Korea, and we sternly warn North Korea to immediately stop its inhumane and low-level actions.”

Pyongyang has previously said it sent balloons south in response to a civilian campaign in South Korea to float balloons carrying anti-North Korean propaganda in the opposite direction.

For many years, South Korean activists and North Korean defectors have sent balloons to the North, loaded with material criticizing dictator Kim Jong Un and USB sticks filled with K-pop songs and South Korean television shows – all strictly prohibited in the impoverished, highly isolated nation.

In a statement carried by North Korean state media earlier this month, Kim Yo Jong, the powerful sister of the North Korean leader, said dozens of balloons, “dirty leaflets” and other material sent from South Korea were again found in her country and near the border.

Despite repeated North Korean warnings, the South Korean activists were “not stopping this crude and dirty play” she said.

“It seems that the situation we cannot overlook is coming,” Kim Yo Jong said in a “stern warning” published by North Korea’s state-run Korean Central News Agency (KCNA), adding there would be “a gruesome and dear price” to pay that could change the South’s “mode of counteraction” with the North.

While the balloons have been going across the border, North Korea has kept up a stream of criticism against military drills by the United States and South Korea on the peninsula, the latest of those being the deployment of US Marine Corps F/A-18 and F-35B fighter jets to Suwon Air Base for joint aerial training this week.

The South Korean Defense Ministry said the US planes will join with South Korean F-15, F-16 and FA-50 fighters in exercises that will end August 8.

A US Defense Department release said the Marine Corps jets were dispatched “to enhance their standard of readiness and lethality with our South Korean allies and joint forces.”

But a KCNA commentary claimed the joint maneuvers were an example of Washington “running high fever in its move to expand the overall structure of confrontation against” North Korea.

North and South Korea have been divided since 1953, when an armistice ended the Korean War three years after the North invaded the South. But a peace treaty has never been signed, so the two technically remain at war.

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